If you know the carrier (you could obtain this info from the user) you can just use SMTP (ie email). So 913 933 3333 carried by Verizon would be 9139333333@vtext.com (or something)
Generally you can look up numbers and figure out what carrier they are. Perhaps someone should build a library to check a number for carrier, and then that could be used to act accordingly. SMTP has to be cheaper than SMS gateways.
That doesn't scale to the rest of the world though. At least here in Australia you're free to port your number to any carrier at any time (I'm on my third carrier with the same number I've had since I was in high school), so you can't use the number on its own to know what network you're sending to. I'm sure this is the same in other countries as well.
