As there are ways of finding the service provider for phone numbers online, then you could probably find a way to get a phone number list, then run it through a service provider finding script, then tack on the appropriate domain to the number, and send a message to the completed list.
You could also have a registration script that automatically converts the number into an email address when the user is first added.
These two options would require some knowledge of PHP, so if you don't know how to use PHP, you might run into some trouble. I've never looked into any service provider-finding scripts before, so I'm afraid I can't help you there.
After a bit of googling...
Drupal (
www.drupal.org) is capable of sending text messages to user lists through a message add-on (found
here). You could check into that and see how that works.
Good luck!!