you can use unlocked iPhones but an upgraded iPhone 5 was introduced that supported better T-Mobile coverage. The 5S once again became universal for the GSM model in the USA.It appears that wifi calling is only activated for the 5S (and maybe not with all plans). I have it and look forward to it. I received afree microcell from T-Mobile a couple of weeks ago but it's not a drastic improvement.
7/8/14 at 5:05am