diff --git a/flowbit-backend/core/serializers.py b/flowbit-backend/core/serializers.py
index 5410cac..2f79f14 100644
--- a/flowbit-backend/core/serializers.py
+++ b/flowbit-backend/core/serializers.py
@@ -1473,7 +1473,7 @@ class RegisterSerializer(serializers.Serializer):
     full_name = serializers.CharField(max_length=150)
     username = serializers.CharField(max_length=150)
     email = serializers.EmailField()
-    phone_number = serializers.CharField(max_length=50)
+    phone_number = serializers.CharField(max_length=50, allow_blank=True, required=False)
     password = serializers.CharField(write_only=True)
     confirm_password = serializers.CharField(write_only=True)
 
@@ -1508,7 +1508,7 @@ def create(self, validated_data):
             is_active=False,
         )
         profile, _ = Profile.objects.get_or_create(user=user)
-        profile.phone_number = validated_data['phone_number'].strip()
+        profile.phone_number = (validated_data.get('phone_number') or '').strip()
         profile.save(update_fields=['phone_number', 'updated_at'])
         user.refresh_from_db()
         return user
diff --git a/flowbit-backend/core/tests.py b/flowbit-backend/core/tests.py
index f223be8..b8956e9 100644
--- a/flowbit-backend/core/tests.py
+++ b/flowbit-backend/core/tests.py
@@ -344,6 +344,34 @@ def test_register_creates_user_profile_and_returns_user_payload(self):
         self.assertTrue(AuditLog.objects.filter(action='auth.register', target_id=created_user.id).exists())
         self.assertTrue(AuditLog.objects.filter(action='auth.email_verification_requested', target_id=created_user.id).exists())
 
+    def test_register_allows_missing_phone_number(self):
+        response = self.client.post('/api/auth/register/', {
+            'full_name': 'No Phone User',
+            'username': 'no_phone_user',
+            'email': 'no-phone@example.com',
+            'password': 'strong-pass-456',
+            'confirm_password': 'strong-pass-456',
+        }, format='json')
+
+        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
+        created_user = User.objects.get(username='no_phone_user')
+        self.assertEqual(created_user.profile.phone_number, '')
+        self.assertEqual(response.data['user']['phone_number'], '')
+
+    def test_username_availability_warns_when_username_is_taken(self):
+        response = self.client.get('/api/auth/username-availability/', {'username': 'AUTH_USER'})
+
+        self.assertEqual(response.status_code, status.HTTP_200_OK)
+        self.assertFalse(response.data['available'])
+        self.assertEqual(response.data['message'], 'This username is already taken.')
+
+    def test_username_availability_allows_unused_username(self):
+        response = self.client.get('/api/auth/username-availability/', {'username': 'fresh_user'})
+
+        self.assertEqual(response.status_code, status.HTTP_200_OK)
+        self.assertTrue(response.data['available'])
+        self.assertEqual(response.data['message'], 'Username is available.')
+
     def test_register_rejects_duplicate_email(self):
         response = self.client.post('/api/auth/register/', {
             'full_name': 'Another User',
diff --git a/flowbit-backend/core/urls.py b/flowbit-backend/core/urls.py
index 8432206..c42407b 100644
--- a/flowbit-backend/core/urls.py
+++ b/flowbit-backend/core/urls.py
@@ -20,6 +20,7 @@
     IdentifierCapacityReportView,
     LoginView,
     RegisterView,
+    UsernameAvailabilityView,
     GoogleLoginView,
     LogoutView,
     MeView,
@@ -60,6 +61,7 @@
 
     path('auth/login/', LoginView.as_view(), name='auth-login'),
     path('auth/register/', RegisterView.as_view(), name='auth-register'),
+    path('auth/username-availability/', UsernameAvailabilityView.as_view(), name='auth-username-availability'),
     path('auth/google/', GoogleLoginView.as_view(), name='auth-google-login'),
     path('auth/logout/', LogoutView.as_view(), name='auth-logout'),
     path('auth/me/', MeView.as_view(), name='auth-me'),
diff --git a/flowbit-backend/core/views.py b/flowbit-backend/core/views.py
index 3c44ace..a74aa87 100644
--- a/flowbit-backend/core/views.py
+++ b/flowbit-backend/core/views.py
@@ -5238,6 +5238,27 @@ def post(self, request):
         )
 
 
+class UsernameAvailabilityView(APIView):
+    permission_classes = [AllowAny]
+
+    def get(self, request):
+        username = (request.query_params.get('username') or '').strip()
+        if not username:
+            return Response(
+                {'available': False, 'message': 'Choose a username.'},
+                status=status.HTTP_200_OK,
+            )
+
+        is_available = not User.objects.filter(username__iexact=username).exists()
+        return Response(
+            {
+                'available': is_available,
+                'message': 'Username is available.' if is_available else 'This username is already taken.',
+            },
+            status=status.HTTP_200_OK,
+        )
+
+
 class LoginView(APIView):
     permission_classes = [AllowAny]
 
diff --git a/flowbit-frontend/src/components/auth/login-help-form-card.tsx b/flowbit-frontend/src/components/auth/login-help-form-card.tsx
index d29ad81..c927959 100644
--- a/flowbit-frontend/src/components/auth/login-help-form-card.tsx
+++ b/flowbit-frontend/src/components/auth/login-help-form-card.tsx
@@ -129,19 +129,21 @@ export function LoginHelpFormCard() {
             setFieldErrors((current) => ({ ...current, requester_name: undefined }));
           }}
         />
-        <AuthInput
-          label="Reply email"
-          type="email"
-          placeholder="Enter the email address for admin replies"
-          name="requester_email"
-          autoComplete="email"
-          error={fieldErrors.requester_email}
-          value={formValues.requester_email}
-          onChange={(event) => {
-            setFormValues((current) => ({ ...current, requester_email: event.target.value }));
-            setFieldErrors((current) => ({ ...current, requester_email: undefined }));
-          }}
-        />
+        <div className="md:col-span-2">
+          <AuthInput
+            label="Reply email"
+            type="email"
+            placeholder="Enter the email address for admin replies"
+            name="requester_email"
+            autoComplete="email"
+            error={fieldErrors.requester_email}
+            value={formValues.requester_email}
+            onChange={(event) => {
+              setFormValues((current) => ({ ...current, requester_email: event.target.value }));
+              setFieldErrors((current) => ({ ...current, requester_email: undefined }));
+            }}
+          />
+        </div>
       </div>
 
       <div className="mt-4 space-y-4">
diff --git a/flowbit-frontend/src/components/auth/sign-up-form-card.tsx b/flowbit-frontend/src/components/auth/sign-up-form-card.tsx
index 264acd6..5c8b6c1 100644
--- a/flowbit-frontend/src/components/auth/sign-up-form-card.tsx
+++ b/flowbit-frontend/src/components/auth/sign-up-form-card.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useState } from "react";
+import { useEffect, useState } from "react";
 import { useRouter } from "next/navigation";
 import Link from "next/link";
 import { FontAwesomeIcon } from "@fortawesome/react-fontawesome";
@@ -8,7 +8,7 @@ import { faArrowLeft, faUserPlus } from "@fortawesome/free-solid-svg-icons";
 import { AuthInput } from "./auth-input";
 import { Button } from "@/components/ui/button";
 import { Card, CardContent } from "@/components/ui/card";
-import { registerAccount } from "@/lib/auth-client";
+import { checkUsernameAvailability, registerAccount } from "@/lib/auth-client";
 
 const signUpNotes = [
   "Fill in your account details carefully before submitting.",
@@ -28,8 +28,42 @@ export function SignUpFormCard() {
   });
   const [fieldErrors, setFieldErrors] = useState<Partial<Record<keyof typeof formValues, string>>>({});
   const [errorMessage, setErrorMessage] = useState("");
+  const [usernameHint, setUsernameHint] = useState("");
+  const [isUsernameAvailable, setIsUsernameAvailable] = useState<boolean | null>(null);
   const [isSubmitting, setIsSubmitting] = useState(false);
 
+  useEffect(() => {
+    const username = formValues.username.trim();
+    setIsUsernameAvailable(null);
+
+    if (!username) {
+      setUsernameHint("");
+      return;
+    }
+
+    const timeoutId = window.setTimeout(() => {
+      checkUsernameAvailability(username)
+        .then((result) => {
+          if (formValues.username.trim() !== username) {
+            return;
+          }
+          setIsUsernameAvailable(result.available);
+          setUsernameHint(result.message);
+          setFieldErrors((current) => ({
+            ...current,
+            username: result.available ? undefined : result.message,
+          }));
+        })
+        .catch(() => {
+          if (formValues.username.trim() === username) {
+            setUsernameHint("Username availability could not be checked right now.");
+          }
+        });
+    }, 450);
+
+    return () => window.clearTimeout(timeoutId);
+  }, [formValues.username]);
+
   function validateForm() {
     const nextErrors: Partial<Record<keyof typeof formValues, string>> = {};
 
@@ -38,15 +72,14 @@ export function SignUpFormCard() {
     }
     if (!formValues.username.trim()) {
       nextErrors.username = "Choose a username.";
+    } else if (isUsernameAvailable === false) {
+      nextErrors.username = usernameHint || "This username is already taken.";
     }
     if (!formValues.email.trim()) {
       nextErrors.email = "Enter your email address.";
     } else if (!/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(formValues.email)) {
       nextErrors.email = "Enter a valid email address.";
     }
-    if (!formValues.phone_number.trim()) {
-      nextErrors.phone_number = "Enter your phone number.";
-    }
     if (!formValues.password) {
       nextErrors.password = "Create a password.";
     } else if (formValues.password.length < 8) {
@@ -119,10 +152,12 @@ export function SignUpFormCard() {
           type="text"
           placeholder="Choose a username"
           error={fieldErrors.username}
+          hint={isUsernameAvailable ? usernameHint : undefined}
           value={formValues.username}
           onChange={(event) => {
             setFormValues((current) => ({ ...current, username: event.target.value }));
             setFieldErrors((current) => ({ ...current, username: undefined }));
+            setUsernameHint("");
           }}
         />
         <AuthInput
@@ -138,7 +173,7 @@ export function SignUpFormCard() {
           }}
         />
         <AuthInput
-          label="Phone number"
+          label="Phone number (optional)"
           type="tel"
           placeholder="Enter your phone number"
           autoComplete="tel"
diff --git a/flowbit-frontend/src/components/support/customer-service-page.tsx b/flowbit-frontend/src/components/support/customer-service-page.tsx
index 4c4b767..5074d89 100644
--- a/flowbit-frontend/src/components/support/customer-service-page.tsx
+++ b/flowbit-frontend/src/components/support/customer-service-page.tsx
@@ -552,8 +552,8 @@ export function CustomerServicePage() {
                 <div className="mt-4 rounded-[22px] border border-stone-900/8 bg-[#f8f6f2] p-3">
                   <div className="space-y-3">
                     {isAdmin && selectedCase.intake_type === "LOGIN_HELP" ? (
-                      <label className="block">
-                        <span className="text-xs font-semibold uppercase tracking-[0.12em] text-stone-500">
+                      <label className="flex w-full flex-col gap-2">
+                        <span className="text-xs font-semibold uppercase tracking-[0.16em] text-stone-500">
                           Reply email
                         </span>
                         <input
@@ -561,8 +561,8 @@ export function CustomerServicePage() {
                           value={loginHelpReplyEmail}
                           onChange={(event) => setLoginHelpReplyEmail(event.target.value)}
                           placeholder="requester@example.com"
-                          disabled={selectedCase.status === "CLOSED"}
-                          className="mt-2 h-11 w-full rounded-[16px] border border-stone-900/10 bg-white px-4 text-sm text-stone-900 outline-none transition placeholder:text-stone-400 focus:border-stone-400 disabled:cursor-not-allowed disabled:opacity-60"
+                          disabled={selectedCase.status === "CLOSED" || Boolean(selectedCase.requester_email)}
+                          className="h-12 w-full rounded-2xl border border-stone-900/10 bg-white px-4 text-sm text-stone-900 outline-none transition placeholder:text-stone-400 focus:border-stone-400 disabled:cursor-not-allowed disabled:opacity-60"
                         />
                       </label>
                     ) : null}
diff --git a/flowbit-frontend/src/lib/api.ts b/flowbit-frontend/src/lib/api.ts
index 7c29cce..36a3ef3 100644
--- a/flowbit-frontend/src/lib/api.ts
+++ b/flowbit-frontend/src/lib/api.ts
@@ -1,9 +1,35 @@
+import { AUTH_TOKEN_STORAGE_KEY, AUTH_USER_STORAGE_KEY, clearAuthCookie } from "@/lib/auth";
+
 const DEFAULT_API_BASE_URL = "http://127.0.0.1:8000/api";
 
 export function getApiBaseUrl() {
   return (process.env.NEXT_PUBLIC_API_BASE_URL || DEFAULT_API_BASE_URL).replace(/\/$/, "");
 }
 
+function isSessionAuthError(status: number, detail: string) {
+  if (status !== 401 && status !== 403) {
+    return false;
+  }
+  const normalizedDetail = detail.toLowerCase();
+  return (
+    normalizedDetail.includes("invalid token") ||
+    normalizedDetail.includes("authentication credentials were not provided")
+  );
+}
+
+function clearStaleClientSession() {
+  if (typeof window === "undefined") {
+    return;
+  }
+
+  window.localStorage.removeItem(AUTH_TOKEN_STORAGE_KEY);
+  window.localStorage.removeItem(AUTH_USER_STORAGE_KEY);
+  document.cookie = clearAuthCookie();
+  if (window.location.pathname !== "/login") {
+    window.location.assign("/login");
+  }
+}
+
 export async function apiRequest<T>(path: string, init?: RequestInit): Promise<T> {
   const response = await fetch(`${getApiBaseUrl()}${path}`, {
     ...init,
@@ -24,6 +50,9 @@ export async function apiRequest<T>(path: string, init?: RequestInit): Promise<T
           : fieldError
             ? String((fieldError[1] as unknown[])[0])
         : "Request failed.";
+    if (isSessionAuthError(response.status, detail)) {
+      clearStaleClientSession();
+    }
     throw new Error(detail);
   }
 
diff --git a/flowbit-frontend/src/lib/auth-client.ts b/flowbit-frontend/src/lib/auth-client.ts
index 193596e..abf306f 100644
--- a/flowbit-frontend/src/lib/auth-client.ts
+++ b/flowbit-frontend/src/lib/auth-client.ts
@@ -140,6 +140,15 @@ export async function registerAccount(payload: RegisterPayload) {
   });
 }
 
+export async function checkUsernameAvailability(username: string) {
+  return apiRequest<{ available: boolean; message: string }>(
+    `/auth/username-availability/?username=${encodeURIComponent(username)}`,
+    {
+      method: "GET",
+    },
+  );
+}
+
 export async function verifyEmailAddress(payload: EmailVerificationPayload) {
   return apiRequest<{ message: string }>("/auth/verify-email/", {
     method: "POST",