Using a Smartphone as an SMS Modem > 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 more messages than this at peak use.