tm htc sensation adding, removing & reinstalling google voice

does anyone know how to temporarily reconnect an old google voice (gv) # long enough to change it's voice greeting and then remove it to reconnect the current in-use gv?

I have multiple gmail accounts that I originally set up for various homebased opps & have google voice (gv) #'s attached to them that still actively recieve voice mails & texts from previous contacts

I really need to change the greetings on them but gv only allows you to change the greeting from a cell phone that the gv is attached to...

in the past when we temporarily reattached an old gv to a phone to change its vm, it locked out the currently in-use # off the phone & while I dont recall how we fixed it It was a nightmare to change it back

any suggestions?