We signed up for SMS.
Was disappointed to learn that applying selection criteria to the SMS list is not yet functional (the option is there, but grayed-out.) This means that the only organic option is to send SMS messages to the entire SMS list.
There is a jump-through-the-hoop workaround. This requires tagging all your SMS records with an SMS tag. Then use that tag to build your segment. Then export the segment, and re-upload it as a List. You can then use that List for an SMS send. That's a lot of hoops to jump through for functionality that should already be there.
Please let us know when we can apply selection criteria to our SMS lists. Thanks!