Rand.Next(Int32, Int32)

[mazanuj]

Your code doesn't include the last phone number from the list Link

[HttpGet]
public IActionResult Get()
{
    var randomNumber = random.Next(0, Phones.Numbers.Length - 1);
    return Ok(Phones.Numbers[randomNumber].ToString());
}

change random.Next(0, Phones.Numbers.Length - 1) to random.Next(0, Phones.Numbers.Length)

[303UsefulIdiots]

I would check how the count is used in the loop, this is written to start count at 0 and might not be the cause of the problem