After the 100 minutes you use on this plan any additional minute is 10 cents. You can always add some extra money in your account and when you need the minutes it'll take it from that.
If you wanted 300 minutes like with VM, you'd have to add an additional $20 on top of the $30. That's quite a jump going from $25 to $50. T-Mo does have a $50 plan with unlimited minutes but you only get 100mb of "4G" internet then throttle to where as the $30 plan you get 5GB.
GrooveIP when I've tried it worked okay in a good 4G area. The person on the other line would just keep mentioning they could hear their voice being sent back on their speaker when they would speak. There are options you can mess with regarding microphone volume levels and other settings but I ended up giving up on GrooveIP as I very rarely go over 100 mins. Maybe other people who've used this app will tell you if its worked for them.
The other app i've tried was Vonage on Android, that one does tend to have some very good quality and is currently free to use. It even uses your own number when dialing out, doesn't work with people dialing in though. Also the free service won't last forever and eventually they will charge by the minute as well.
Honestly this plan is incredible considering the options on prepaid now, sadly T-Mo only stuck 100 mins in this. Guess they didn't want to undercut their higher up plans.