I stumbled upon this HN discussion of about 2 years old http://news.ycombinator.com/item?id=1996970<p>Want to know fresh thoughts from people using different services recently?<p>Metrics are Price (including international calls), reliability, support/docs, and anything else you like about a service.
twilio is the entire package. If you use OpenVBX along with twilio you can replace your phone system. And if you still want to hold onto your phone system you can use SIP out with twilio. Pretty soon they'll have SIP in as well.<p>Overall, twilio is more robust, more reliable, priced better, and more flexible. No one in the market even comes close. I've been doing telecom for over a decade and there's nothing better than twilio for telecommunications.
Definitely biased here, since I work for Voxeo Labs, but Tropo is free for development, supports SIP, SMS, Shortcodes, TFN's ITFN's, ect. We also have great support ( 24/7 ) and fantastic documentation. Our product is also available on premise, should you want to go that route. It is also being deployed into many large carrier clouds, such as AT&T, Deutsch Telekom, ect.
Plivo. I use it to provide voice/text services for my personal line and I couldn't be happier with them. I think the price is slightly cheaper than Twilio and you get a similar set of features. I've run into the occasional quirks here and there, but I've always been able to get great support right away.
At Babelverse, we first used Tropo's APIs to hack together a crisis-response tool - literally overnight - post-Japan earthquake & tsunami. We've been quite happy with it since then.