I just went through the process of looking at three VOIP companies (Vonage, AT&T CallVantage and Verizon Voicewing) and picked CallVantage. Everyone's situation is different; I don't make enough long distance calls to justify a plan with unlimited LD calling. But I wanted a lot of features, especially Call Filtering where I can block callers without Caller ID as well as 20 specific numbers. I read a lot of reviews on the three companies, and while Vonage has the most customers, they also seemed to have the most complaints about the service. Vonage has a $25 plan with unlimited LD and a $15 plan with a per minute fee for LD. CallVantage has a $30 plan with unlimited LD and a $20 plan with 4 cpm LD. Voicewing has a $35 plan with unlimited LD (can't remember if they offer a per minute LD plan). I chose the $20 CallVantage plan, adding $2 for call filtering, total monthly cost with taxes and fees $27.
Voicewing claimed that only one phone can be connected to their telephone adapter, and if one wanted multiple phones, they would have to purchase a multiple phone set. CallVantage claims that one can use the existing house wiring once the incoming landline is disconnected, and all phones can be used. Another reason I chose CallVantage. I think this has more to do with the REN number (Ring Equivalence Number) of the phones, where an REN of 1 is the equivalent of an old style non-powered phone with a ringer that is powered by the incoming phone signal. With nost cordless phones that use house power, the REN is about one tenth of an REN, so multiple phones can easily be hooked up to a telephone adapter without loading it down.
The only negative I have found so far with CallVantage is that the telephone adapter is very sensitive to power glitches. When the TA detects a power flicker, the next outgoing call made connects to an AT&T service that asks me to verify that the phone has not been moved (by pressing 1). This is done to comply with the E-911 regulations. I have the telephone adapter connected to my computer's UPS, but the TA still sees an occasional flicker. Fer cryin' out loud, I couldn't possibly move the TA to a new location in a few milliseconds. Wish they would have designed a full minute in to the power outage detection circuitry before sending me to the location verifier at AT&T.
One problem I ran into trying to transfer my home number to VOIP is that AT&T did not have a portability agreement with the rinky-dink phone company that I had. Even though I was paying the small fee every month for number portability, AT&T could not get my number. So I am using the phone number assigned to me by AT&T, and am in the process of porting my number to Verizon, after which I can port my number to AT&T.
HTH.
Lena