I've been playing around with Twilio for the past two days and I could already recommend it. It took me about about two hours to get it working well enough to make my own phone ring using their API. And of that the second hour was because I was confused, my number is international and it needed special permissions to call that. Now after two days I know how to make recordings, set up conference calls etc., so easy and powerful. I have a toll-free number in the US now ($5/month, $0.05/minute) just because I wanted to play with one.