Plivo is a free and open source communication framework for creating voice based applications that give you telecommunication features such as make and receive calls from any phones, Skype and Gtalk or send SMS to applications.
By using this framework, developer can create voice based applications such as IVRs, Billing System, Voicemail, Click to Call etc.. which directly interact with phone system using your existing web development skills and infrastructure to program custom business logic (via HTTP) that can control incoming and outgoing phone call instances.
Plivo is a 100% FOSS framework which completely built on the open source telephony platform FreeSWITCH and is written in Python, gevent and Flask.
plivo framework is perfect open source alternative for Twilio. Also FreeSWITCH provide native support for Skype, SIP, H.323, Google Talk and Google Voice means that applications developed by Plivo can easily compatible with these platforms.
Download : Plivo communication framework