My company moved to Plivo from Twilio. We've used them in production for 6 months or so now and they have saved us a ton of money. Their support is great.

They have a gem and it's pretty simple to replace Twilio, there are some minor changes in attribute names and such but all the functionality is there.

Plivo sip implementation is great, played with their API, on a few side projects and their documentation is straight forward.

Support is excellent, highly recommend their sip API. Especially since they are showing devs a ton of love,

http://blog.plivo.com/post/45362689649/free-plivo-developer-...

Works very well too. Plivo also supports SIP Out...outbound calls from SIP devices and registering SIP devices to their system.

The problem with this is the 0.004 per minute charge. A US call cost from 0.007 and up (flat rates) this will only save up 0.001 to anyone who will use SIP.

Then there's this issue with Apps developed in Plivo cloud not being that much 100% compatible with the open source plivo framework.

Similarly, Tropo has long had support for inbound and outbound SIP.

http://blog.tropo.com/2010/03/09/does-your-cloud-provider-of...

(Disclaimer: I work for Voxeo Labs)