Here is a story from using the Plivo APIs, that convinced me that these guys are for enterprises and serious businesses.

We were looking at different telephony APIs to build a highly concurrent calling system. With Twilio, we could only send one phone number in a single http call. That was a bummer. With Plivo, we could send as many phone numbers as a http POST can handle, and let their APIs handle the high number of calls per second. We were pleasantly surprised that they do so much of the heavy lifting.