Start is fairly new to the VoIP game but it does get pretty good reviews on other forums. I guess they are planning some enhancements this summer.
I am with Start cable internet but when I signed up they didn't have VoIP. I signed up with Teksavvy's basic $10 VoIP service and so far I'm loving it. Works great on the Start network. I would assume Start uses the same equipment as Teksavvy so I would think Start's VoIP would be great too.