I personally like http://www.tm4b.com http://www.tm4b.com Never used it in anything with enough volume to care about a cent or two here and there, the API is e

I personally like http://www.tm4b.com http://www.tm4b.com Never used it in anything with enough volume to care about a cent or two here and there, the API is excellent, and incoming messages work a treat too. Related: I also like http://tringme.com/ http://tringme.com/ for actual calls, although their documentation and support is terrific (or was when I checked), their service is pretty good once you understand how the hell to use it.