Before investing any money I would do the following;
It's free to establish a gv number, so go ahead and create one. You'll need to provide your cell # (or any other active number) for verification.
Once established, you can configure an unused sp slot to use it. This way you can fully test to see if gv service is to your liking. We did this for about 3 weeks before moving our att pots line over to gv.
Keep in mind the following caveats.
1) Google voice does not support e911 (you can forward to a free callcentric DID and pay $1.50 a month for e911).
1a) You can use anveo's e911 service which costs more ($25/yr?) but can be configured through obitalk
2) Google voice only provides the number portion for caller id
2a) Callcentric's free DID's provide cnam caller id. Establish a free callcentric DID, then add it to your gv account as a forwarding number. This works well with their e911 service.
Costs: Google charges $20 to port in a number. In addition to the sim card fee, most providers will require you to apply some sort of refill to the prepaid before porting a number in. This was the case with att prepaid. Tmobile might be different and/or rules have changed now.
Yes it's a convoluted process, but the end result gets your number into the GV system.