Before you evaluate vendors, if you are going to offer VOIP, decide what level of quality you want to offer your users. Then look at the vendors. Burps in VOIP (pauses when you cant hear the other person) can annoy users and reflect badly on your offering.
Genband's Kandy.io should be on the list. Tropo (Cisco bought them) and Nexmo are others I'd include in the evaluation.