For VoIP I would recomment finding a VPN solution that uses UDP for it's transport medium instead of TCP (personal bias). As far as just a router goes something like a Linksys RV-042 or Cisco ASA should work if you want a commercial brand. If you like opensource then I would recommend Untangle or Vyatta (the latter being the more "serious" of the two). I'm afraid I don't have much experience with watchguards, fireboxs, sonicwalls, or barracudas.

The only other thing I will mention is to turn off SIP helper applications on the routers. They almost ALWAYS break SIP despire their misleading name. Cisco calls it "SIP Inspection". Others call it different things.