You can technically hack together your own equivalent of UMA calling utilizing a VoIP carrier, but then you run into issues with various MVNOs that may not permit call forwarding, like Platinumtel.
Google Voice would be one way to do it and you could just utilize something like SipDroid or
Talkatone to bridge the gap while in bad coverage areas where WiFi is present, but there are security risks due to how password management is handled with your Google account credentials, especially if you deal with a public WiFi network and a few other concerns regarding quality and reliability that you appear to already be familiar with. Normally it works, but when it doesn't... brother it
doesn't.
Another way would be to utilize a VoIP carrier like
Future Nine with the
3CX or
SipDroid, or the Android integrated SIP clients who allows you to
set your wireless number as the outgoing CID number for making calls. This way, if you don't have GSM service on your handset, you could just forward your number to ring over to an incoming VoIP number like
F9's Deep Discount plan. As you can tell, it's technical but doable and not as difficult as you might think to set up on your own. Depending on the VoIP provider, you can swing between anywhere from free to cheap, but it'd dependent upon your MVNO carrier to allow you to forward calls if you wanted to have relatively seamless incoming as well.
Unfortunately, Platinumtel does not provide that. It's the biggest and only real caveat I've ever had with them. That said, Airvoice does permit call forwarding at the same rate as incoming calls... so incoming calls to your cell phone would still cost minutes.
Alternatively, you could always install an active signal repeater as T-Mo doesn't provide femtocells, but those are expensive ($300 or more) and require an FCC license to operate now. There's also passive antennas/repeaters, but they're not very good and typically designed for cars.
Unfortunately, there's just really no perfect solution for UMA calling unless the carrier supports is directly, and you'll get billed for it.