Text to speech is magical. This technology enables us to type out text and get the computer to read (or say) it just like a human! You will encouter Text to speech at various places in exotel. In particular, you would use TTS if you chose the "Read text" option in say, the zero app or the greeting applet.
Exotel currently uses the free & open source Festival Text to speech engine presented to the world by the Center for speech technology research at University of Edinburgh. This engine needs to be powered by a speech synthesis voice library which, for our understanding, is like a dictionary of all the possible pronunciations spoken by a single person packed as a library.
Through trial and error (If you are keen to know what this trial & error is, Our product manager sent out a survey to everyone inside Exotel along with synthesized audio files from various paid & free TTS engines. For archive & fun purposes, this is the survey) we found that the HMM based Synthesiser built by Carnegie Mellon University to be most understandable by an Indian audience.
To get an idea of how exotel's TTS will sound, listen to this SLT voice reading an excerpt from The Mystery of Chimney Rock
While we found this American female voice to suit our Indian listeners best, it is still American accent and it is robotic - So, most people in India might not understand it while on a phone with background noise & flaky network signals!
Until we get our TTS highly suitable for Indian audience (If you have suggestions, Please send it to us!), Consider using our TTS only when testing your app (Typically on a trial number) or when you don't have a choice (Like reading numbers that the user has entered!)