Great platform for voice and SMS applications
Twilio is not a traditional carrier but a complete platform for telephony applications. Their offering is squarely aimed at the very technical person and should not be anyone's first choice for simple VoIP service. As far as standard carrier functions like porting are concerned, Twilio is no-nonsense and virtually everything is self-service. (If you do need to engage their customer support, be aware that their turnaround time is generally pretty slow.) Once phone numbers are on Twilio's platform, they can be associated with URLs--either on Twilio's servers or your own--that belong to applications written using Twilio's API. It's possible to build anything from a simple IVR system, to an SMS chatterbot, to a fully fledged real-time voice application that integrates tightly with an existing web application. There are other VoIP provides that offer sophisticated features, but what makes Twilio unique is the ability to write and host your own custom applications, while leaving the complex telephony side of the equation in Twilio's hands. If you can code it, you can pretty much make it happen.
Great platform for development of telephony apps
As lot of reviewers mentioned Twilio is not a telephone services company but robust platform for development of telephony applications. Geared for developers and corporations with in-house dev teams, Twilio allows easy development of applications which would otherwise be out of reach of many firms. Porting a number is a breeze and customer service is very responsive.
Though not everybody's cup of tea, Twilio can be your best bet if you know how to work it.
Very happy with Twilio
I've been using Twilio for various voice related projects and never had any issues. I have full control over all aspects of processing calls. Twilio has created a really nice eco system for voice related projects.
Twilio is stupid simple. When I ported my number over, they gave clear step by step directions, along with a ton of status updates when I needed to take action. They are pro and I don't know why you would want to use anyone else (outside the cost)
Easy to use service for developers
Twilit is awesome. Their processes are clear and easy to understand, and the number porting process was fast and simple. We're very happy with Twilio.