What kind of reliability and quality is available with VoIP? In particular with using Comcast as a service provider.
Since in case of voip the traffic flows through an IP network one major factor is the quality/reliability of the network itself and the other is the service provider. If you connect through the Internet (instead of a dedicated circuit for instance) then nothing is guaranteed.
If you have a good network (qos enabled, enough bandwidth, redundant, stable etc.) the voice quality can be very good (if you use g.711 uncompressed codec the voice is almost the same as with traditional telephony). I have no knowledge with Comcast so I propose if you seriously thinking about to use them let's ask for a trial period.
Hope it helps,