GrooveIP ($5). I use this as my backup if my cell service goes out.
I have this installed on my Droid 1 with NO cell service and I can make and receive calls using my google voice . It does work as advertised. The quality is not as good as real cell service but it fairly decent. Better than I expected.
I will warn you that this will involve occasionally jumping through hoops. It is not as seamless as real cell service although it does work fairly well. GrooveIP will work in place of your dialer and will call and ID numbers from your GMail contact list.
That being said, I think you should reconsider upgrading. The Droid was a great phone, but there is a night and day difference between it and the new phones. The response times are immediate...I do not ever wait for apps to load anymore. You can get a Galaxy Nexus for $400 RETAIL (no contract) and use Groove IP with it to make and receive cell calls.