T-Mobile's prepaid plan does not support unconditional call forwarding, which is required to use Google Voice as the voice mail box on a non-Google Voice number.
However, just using a separate Google Voice number and its voice mail box works fine.
I have the same plan ($30 Monthly4G) I love it (no fees and taxes) except for this lack of unconditional call forwarding.
I don't think it is required to use google voices voicemail.
While the google voice app on the phone won't be able to setup voicemail (cause of lack of conditional call forwarding), you can activate it through google voice desktop version. As long as your carrier voicemail is disabled (I called them to disable it, and have them turn off web-guard), then when someone calls your google voice number, it will forward to your phone, but if no one answers the phone, after x seconds your google voice voice mail should pick up the call and your phone should show a missed call, and a google voice notification (for the voicemail).
I'm not sure why the OP is having problems. Lots of people have it setup this way.
So I'm a little bit confused about what you guys are saying. Is it possible to set up voicemail through Google voice on T-mobile's monthly4G plans? If so could someone put up a guide or something? I added my phone number to my Google account and got to the point where I could enter the code as a regular phone number (the MMI code) and my phone gives me the error saying call forwarding isn't supported. TIA
Check the other $30 prepaid thread for more info.
Basically if you just want to use GV for Voicemail and not calls, you have to call and tell T-Mobile that you'd like to cancel your T-Mobile Voicemail service. Then you'll be able to set up GV for Voicemail at that point.
The issue is that the way GV normally works for Voicemail is your T-Mo # receives it then forwards to GV #. Most prepaid plans don't offer call forwarding so it won't work.
Of course if you're all-in and ported or using GV as your primary number, then you just use that and GV handles Voicemail normally.