I am doing this, and yes, I bought a $20 phone from walmart to get the sim, and yes wifi calling won't work.
You can also get TMO at their 800 number to sell you and mail you a sim for $5 that will work. Just not retal store sims.
It works well, except that TMobile misled me, the TMO operators at the 800 number told me repeatedly I could tether as much as I wanted up to all 5Gb on the $30 walmart plan, but in reality, it is only good for the first 200MB, AND EVEN SO, LAST TIME I TRIED IT IT WASN'T WORKING.
They have an app called tethering manager which seems to be malfunctioning on the SGII as it is now with the unrooted ROM, that prevents a lot of people from tethering.
Apart from that, it works fine.
If you could find someone who has it cheaper than get it I have the same phone on the $70 plan and I love it but I hate my visual voicemail doesnt work that sucks but if you dont want yo keep paying full retail price for your phone call Ensauared phone insurancr at 18884067925 they insure all/any phones
So I figure you all might be interested in hearing about how this all worked out. I got T-Mobile for $30 a month on my Galaxy S2. Only problem was t-mobile didn't have very good coverage in my area.
I also tried Straight Talk and Go Phone. Right now I think I'll go with Go=Phone (AT&T) I can get voice for 10 cdnts per minute and 0.5 Gigabytes data for $25. Coverage is good cause it's AT&T. 0.5 Gigabytes is a lot if you don't watch movies or stream hour of music.
hopefully Go-Phone will work out.