Here is your answer. People have a tmobile phone number and give it to someone. Not many people are going to give out there phone number and/or a Google voice number. If someone calls the phone number it will go to T-Mobiles voicemail not Google's. In order to have you voice mail service send them to Google voice instead you must have call forwarding to allow your phone to do so. With flex pay there is no call forwarding so this can't be done.
Why not just use T-Mobile voicemail ? Because googlevoive is different. It allows users to read a transcript of the voicemail directly on the phone. This works great when making a call is not an option. Also you can check voicemails online and easily store them.
I hope this helps.
Nope, maybe you missed my question:
You can do/use all aspects of Google Voice (with FlexPay) as I do it everyday. The ONLY thing you can't do, is use your GV voicemail box instead of your TMo voicemail box. If someone calls your TMo #, and you don't answer, they are going to get your TMo VM. What's the big deal?
I asked my question to the OP because the title of his thread read "Why can I not get Google Voice to work?" and then the thread went in the direction of a voicemail feature that he may be misunderstanding and not realizing that's not something he wants anyways. It sounds like he has 500 minutes, and doesn't want to use them so he would rather use GV, which is seamlessly integrated into the Nexus, works awesome, and has nothing to do with your plan.
I don't mean to offend the OP if I am wrong, but he sounds like he's having difficulties setting up GV; maybe he's not super-techy and is misunderstanding the feature and not intending to eliminate his TMo VM and use GV VM in it's place. If I'm correct, it's totally understandable because the verbiage in the "settings" section makes it a little confusing.
Google Voice works with the Nexus, works incredible, and incredibly easy. I use flexpay myself so there's no surprise charges or fees at the end of my month (like the huge bill I would have racked up not knowing we get charged to call a Canadian #. Instead it told me I had to add funds to place the call, so I use GV: free to Canada). My TMo # I ported over, have had it forever, and everyone knows it. I use my GV # for work and advertising, so that I can set it so after-hours calls to go VM instead of waking me, and it I ever needed to change the # I don't have to tell everyone else my new #.
When I dial and hit send, I'm prompted to use my "minutes" or my Google number. If I miss a call to my Tmo #, it goes to my TMo visual voicemail. I miss a call from my Google number, it goes to Google Voicemail and I can play it same as my Visual Voicemail.
OP: Did I understand you correctly? If so, what are you trying to use Google Voice for, and let's help you get it set up correctly.