Yahoo IM has all carriers, yet, if they have say 5 cent per text, it is still 5 cent per text. It's not free for them, only the person on the phone. If they have unlimited... who gives a shit?

Way you do it...

Add them on Yahoo. Anything really. You don't actually have to have anything. You could put Add Contact > A@yahoo.com > add.
Then... Right click A@yahoo.com(after it "adds" it) and click send SMG. Enter phone number. Tadaaaaaaaaaaaa