Obviously that's not the hard part. Building your own Twilio is. I want to depend on multiple service providers, like you can do with Asterisk or FreeSWITCH. When X provider fails use Y provider. When Y provider is cheaper use Y. That's the true power of openness which OpenVBX has a vested interest in not providing.
Automobile manufactures will tell you to use factory parts, too. Doesn't mean there aren't other compatible options.
OpenVBX would run on any platform that emulates Twilio's API. That includes Twilio (of course), Plivo, an Teleku (disclosure: now owned by us). There may be others out there as well.