VoIP technology through VPN?

I’m sure this has probably been discussed many times. I will also do a research in addition to this….

My question is what do you people think about running VoIP applications through a VPN connection? We have a site with a full T1 to the Internet. One with a Fractional-T1 of 768k to the Internet. Latency between the two site is 120ms – 130ms on average. We have 30 phone users at both locations. We are planning on install two new IP-capable switches or phone systems. Will the quality of voice going over this VPN be acceptable, poor, or totally a bad idea? Who has done this route and how is it?