Twilio is a full hosted phone system (VOIP), and exposes every single feature using a documented API.
The Documents section has code samples, ideas and suggestions to use Twilio for different purposes.
This is not a service oriented to end users, and requires a good level of technical skills to develop a solution, but it's very flexible and you can create anything imaginable tied to a voip system. I see Twilio as a Developer oriented platform, to create awesome solutions for your own customers. (as Twilio says "We empower developers to build powerful communication").
The pricing levels are fine, but there is no "unlimited" phone calls package. Its possible to implement a full phone system: IVR, SMS, Phone Menus, , integrated with you iPhone or Android device, or your SIP system.
I really like the option of purchasing phone numbers directly on the control panel. This numbers also can be located in several US states even countries around the world.
A free PBX software named OpenVBX can be used with a Twilio account to create your own phone system, using a minimum of hardware and software.
Another highlight, are the "sandbox" credentials, to test your projects without expending your credits.
They don't offer a unlimited minutes package for phone calls (land lines), you need to pay by the minute inbound or outbound. This makes some phone time intensive projects too expensive to be implemented with Twilio.
So far, this is the most "open" cloud phone system I've seen. I tried to work with other platforms, but so far this is the only one that provides a good deal of features a reasonable price.