[ze_dude]

http://www.twilio.com/

I haven't used them in production, but I've started playing around with their voice API and it's pretty neat.

Also, you get $30 credit (for free) when signing up for a test account, so that should be more than enough to fiddle with tests and get your integration up and running.

[steverb]

Twilio really does rock. Super easy to use and get started and there are other providers that provide Twilio API compatibility in case you ever need it.