You can ask T-Mobile to disable their voicemail. I was able to do it through chat support. Then voicemail is always through GV.
EDIT: This assumes you have a separate GV # and are forwarding to T-Mobile.
Months ago when I was thinking of switching to T-Mobile from Sprint, I was looking at the T-Mobile pre-paid plans. With Sprint I used Google Voice just for voicemail (sometimes called "Google Voice Lite"). During this research, I discovered that T-Mobile prepaid plans didn't do conditional call forwarding, which is required for Google Voice Lite. That may have changed since I did this research, but this could be your problem. I have regular postpaid T-Mobile and Google Voice Lite works fine.
Hi. Is there a tutorial floating around for using Google voice for voicemail with the N5 and T-Mobile prepaid? Right now it's defaulting to T-Mobile voicemail.
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I also have a T-Mobile prepaid plan. My T-Mobile number is one of my forwarding phones in GV and when I tried to Activate Google voicemail on it it failed because of Conditional Call Forwarding not being supported by T-Mobile prepaid accounts. Are you saying that if I have T-Mobile disable my voicemail with them (even though I haven't even set it up yet) then I won't need to worry about Conditional Call Forwarding support (or lack thereof on the prepaid plans)?This is correct. If you use the standalone Google Voice number and forward calls to your T-Mobile prepaid line, you can use Voice for voicemail by disabling the T-Mobile voicemail feature altogether. Any calls coming through your Voice number will be picked up by the Voice voicemail if one of the forwarding lines does not answer the call in a set amount of time. Unfortunately, if one of the forwarding lines goes to voicemail, this counts as 'answering' the call.
I also have a T-Mobile prepaid plan. My T-Mobile number is one of my forwarding phones in GV and when I tried to Activate Google voicemail on it it failed because of Conditional Call Forwarding not being supported by T-Mobile prepaid accounts. Are you saying that if I have T-Mobile disable my voicemail with them (even though I haven't even set it up yet) then I won't need to worry about Conditional Call Forwarding support (or lack thereof on the prepaid plans)?