For people who want to use Google Voice Mail with your T-Mobile Pre-Paid phone as your primary number, sorry I have no answer for you. This is for people who want to use a Google Voice number as a primary contact number and want to keep the TMO voicemail from answering before GV voicemail activates when the phone is switched off.
I use my Google Voice number as my primary number, it's the only number I give out to people. Google voice forwards to a prepaid T-Mobile cell phone. If I don't answer Google Voicemail kicks in after 30 seconds of no answer and that was just quick enough to avoid T-mobile voicemail answering if the phone was turned on. My problem was that if the phone was off it goes to voicemail after only one ring so those message end up on T-Mobile voicemail. I tried to get T-mobile to disable voicemail so this would never happen. Many people on the net say they have done this but the agent I contacted said she could not. Maybe if you get the RIGHT guy in technical support, but I didn't, and I hate to keep calling back with the same issue.
Someone did however clue me to what seems a brilliantly easy workaround. I recorded a custom TMO voicemail greeting, 30 seconds of dead silence. I was skeptical that this would work at first. One guy on line said it didn't work for him (I don't think he had TMO) but it seems to work for me. I guess GV listens for a voice to know if you picked up so 30 seconds of silence in effect defeats the TMO voicemail. If you want to try this do not use the ?Record Name as Greeting? options since a female voice will say ?Please leave your message for? at the beginning. Instead use the Record Personal Greeting? option and give it 30 seconds of silence. If you did it right when it plays your greeting back you will get 30 quiet seconds followed by the little bleep. You can also try calling the phone directly to see how it sounds. Do a full test by turning your cell phone off and calling your GV number from another line.
If this works reliably it has the advantage that you can do it yourself and easily reverse it later, no depending on TMO agents who may or may not know how to meet your need. Hopefully some people will post their experience.
I'd like to know if others have had this issue.
Did you get T-Mobile to disable voicemail for you and how did that work out?
If you try this I would love to know if it works for you, is it really reliable or not.