said by nickz :

Oh and I just thought of one other suggestion: If you go Vonage and decide to port your number to them don't fax in the authorization form until you are sure that VoIP is going to work for you. This way your VoIP line will have the same number (and caller ID) as your POTS line for outgoing calls. (Only incoming calls from other Vonage users will actually ring your Vonage line at this point--you can give out the temporary virtual number to people so you can test incoming call quality.) If you decide you don't like Vonage and want to try another provider or stick with POTS, all you need to do is cancel and your original phone number will still be routed to your POTS line. Fax them the transfer authorization only after you are satisfied.