Why can SMS recipients not respond to messages sent using an alphanumeric Sender ID? MessageBird October 01, 2018 13:37 Updated Follow For a phone to be able to send a response to a message, it needs a mobile number to respond to. When you send out messages with an alphanumeric sender ID, the receiving phone does not have any mobile number to reply to. Related articles How can I receive SMS from my customers? Who pays for any incoming SMS messages? How do I manage cancellations (opt-outs) from my mobile number database? Forward SMS responses to my mobile phone Forward SMS responses to a URL