I'm on T-mobile's $30/month prepaid plan for unlimited texts and 4G data (you get throttled to EDGE speeds after 5GB but ... 5GB is a lot.) Voice calls are "limited" but that's what VOIP is for!
Unfortunately this plan is "web exclusive" and limited to new activations only, so you have to order a new SIM online and activate it with a new number to switch to it. (You can port the number afterwards; I just route everything through Google Voice).
I've tried Talkatone, Sipdroid, and Groove IP. GrooveIP is easily the best (sadly). Tighter integration sure would be swell, but it seems a lost cause. It works well enough, especially for receiving calls while on Wifi. Beats the hell out of paying for minutes, though.
Unfortunately this plan is "web exclusive" and limited to new activations only, so you have to order a new SIM online and activate it with a new number to switch to it. (You can port the number afterwards; I just route everything through Google Voice).
http://prepaid-phones.t-mobile.com/monthly-4g-plans