No SMS provider = email only. If no phone route is Ready, customers are offered email codes only, and Verify at signup: Phone cannot be completed. Set up SMS before requiring phone verification.
DLT missing. Codes "send" but never arrive on Indian numbers. Add the DLT template id on the SMS channel page.
Too many requests. After the allowed number of codes in the resend window, more requests are refused until the window passes. Explain this to customers who keep pressing resend.
Shared office or mobile network. Many customers can share one network address. Keep Codes per network address per hour generous.
Foreign numbers. If you restricted Countries that can receive codes, customers abroad can't receive SMS codes; they can still use email.
One phone, one account. A verified phone number can register only one account.
Changing a setting mid-day applies to the next code; codes already sent keep their old validity.
