What happens if your recipients reply to an SMS with an alphanumeric sender name?
If you send your SMS with an alphanumeric sender name, the name will be displayed to your recipients when they receive the SMS.
Generally, your recipients should not be able to reply to this type of SMS.
Unfortunately it is possible for recipients using iOS to reply to this kind of SMS. However, the reply will NOT be sent to your account, but to a number that is the T9 equivalent of the sender name. So if your sender name is ABC, the iPhone will try to send an SMS to 123 when your recipients reply to your message.
Unfortunately, we can't influence this behaviour because it's a technical limitation on Apple's side.
If you want to make sure that your recipients don't reply to SMS with alphanumeric sender name, you should explicitly mention this in your SMS and offer alternative ways to contact you if necessary.
You want your recipients to be able to reply to your SMS directly? Book your own inbound number to receive SMS reliably.