I've wanted to play with Twilio's voice and SMS service for a while and finally got the chance at an all night hackathon.
Twilio is almost perfect. Very easy to use, well documented and generous with a self service free trial. They give you enough rope to fully build out a client before you need to think about paying them.
I gave up on AT&T's platform after navigating a bazillion forms before discovering they wanted $99 before I could even get a taste.
The one hitch with Twilio is that the voice transcription they offer is appalling. Just absolute gibberish. Unless it's just my accent, or the hackathon whisky. Seems to be a common complaint though and so to take my IVR app further I'd need to bolt in another solution. A pity given how much Twilio gets right.