That formula would be for if you wanted this to have the ability to send to multiple carriers and numbers.
If you only want to send it to a single number and carrier then you don't need a formula at all. You could just enter it in the notification recipient and send it as an open email(specific list of users).
5555555555@txt.att.net should work, no dashes or anything else.
You might have to contact QuickBase support and have them open your account up to allow you to send emails like that. Out of the box I believe it will not allow you to send emails to anyone, just registered users. Unless they changed something.