I recommend this IP telephony service. The choice of plans, of course, depends on your needs, but the most suitable one is the "Corporation" plan. We use it for a small call center. Connecting to the CRM took less than half an hour, following the instructions; the integration is free. The call quality is high, there are occasional disruptions, but that's more of a rarity; we haven't had any serious issues in six months. This is an example of how something inexpensive can also be of high quality.
