Using a Smartphone as an SMS Modem
The term "smartphone" refers to any mobile phone that is sufficiently feature rich that it starts to resemble a little PC. In this section, we'll be taking advantage of smartphones that run the Windows Mobile 5 or Windows Mobile 6 operating system, because they expose APIs that you can use to send text messages programmatically.
Using a smartphone as your SMS server can be a cheap and easy way to get true SMS connectivity: all you need is a Windows Mobile smartphone, a PC to connect it to and (ideally) an unlimited text messaging plan from your carrier.
Think of your phone as an SMS modem attached to your computer. All the logic for sending and receiving the messages will execute on your Windows PC, you'll just use the phone to actually send out messages and receive messages on your application's behalf.
Advantages and Disadvantages
Using a smartphone gives you the addressability of an SMS Aggregator (i.e., you get a real phone number that you can give out for your users to send messages to) without the high set-up and per message costs. Pick up a used smartphone on E-Bay for a couple hundred dollars, and shell out $20/month on an unlimited SMS plan, and you're in business.
In business for a while, anyway. Scale will likely be your biggest limiting factor when using a smartphone-based SMS solution. In our experience, a single phone can process only 30 messages per minute. If your service succeeds, you may wish to support orders of magnitude ...