The source SMS addresses generated by the MTA SMS channel should be set to match the selected SMS address prefix. Do this by setting the following:
MTA SMS channel options:
The first setting causes the channel to not attempt to set the SMS source address from information contained in the email message. The second setting causes the SMS source address to be set (to the selected prefix) when it is not set from any other source.
Recognize the prefix as an SMS destination address to accept and route to email. Do this by specifying the SELECT_RE gateway profile option as follows: