How do I enforce users enter their phone numbers with a leading '1' (trunk prefix) when enrolling?

It will accept their phone number without it, then when they attempt to unlock an account the SMS will error out due to the missing '1'.

The only option I see is to set the format to XXX-XXX-XXXX - which does the opposite, and does not allow them to enter the 1 at all.

Or is there a way to have the SMS gateway just add the 1 if not present to all?

Thank you!