This technology cuts down your phone bills. To get the best out of it, you choose a service provider that will give you the lowest rates without sacrificing the quality. Regarding phone number transfers these concerns are known by the carriers and have been addressed by the Federal Communications Commission.
For the voice quality, it does not depend on the VoIP provider alone. Check your broadband connection for the speed. Voice quality also depends on the speed f your connection. You can check your broadband speed test and VoIP at myspeed.com. They are able to generate a report. It is easy enough to understand without using much of technical terms.