Yep, no conditional call forwarding on T-Mo prepaid, thus no GV voicemail for the handset number. Basically, you have to go "all-in" with GV.
Straight Talk (AT&T or T-Mo MVNO, you pick the SIM card) does conditional call forwarding and I believe Solavei (T-Mo MVNO) does as well. As far as the other prepaid MVNOs are concerned, I'm not totally sure.
Straight Talk is $45/mo for unlimited talk/text and "unlimited" web (they don't clearly define what "unlimited" really means and they can cut you off if they find your data usage "abusive"). $15 for the SIM card, I think. You pick an AT&T or T-Mo SIM card (whichever network is better for you) when you sign up.
Solavei is sold through an MLM (multi-level marketing) platform. You don't necessarily have to sell anything and you can simply be a "service-only" member if you want. $49/mo gets you unlimited talk/text and 4GB of high-speed HSPA+ data (throttled to 2G/EDGE thereafter). But they do charge $49 for a SIM (but sometimes run promos that waive that fee, I hear) and there's a $49 activation/enrollment fee as well. But in the long term, it seems to be a pretty good deal. I've been pondering switching from T-Mo prepaid (I'm currently on their $60 plan) myself. The other catch: you have to be "sponsored" by another Solavei member to enroll.