SMTP SMS gateway wildcard

Hello, I am trying to send SMS verification codes via SMTP. I've got it to work in testing just fine by putting %mobNo%@txt.att.net in the "To" field, but I'm wondering how I can change that "@txt.att.net" to a variable or wildcard so if the user has a Verizon phone or Tmobile phone, it will also be sent to them? This could probably go in the feature request forum. Seems like you could have an "SMS Provider" field in the user's inforamation, and then have the big cell carrier's default SMTP to SMS addresses, and then have the software send to the right carrier based on what is selected in the "SMS Provider" field.

But for now, is there a way to not be specific with who to send an SMS verification code to?
Thanks, Phil Duncan