According to what I read, the SIMS are interchangeable.
"So basically, PTEL uses T-mobile network. and the device screen shows "T-Mobile 4G""
http://www.howardforums.com/showthread.php/1795094-Tmobile-Mobile-Hotspot-with-Ptel-Simcard
I am good till next year, so no hurry unless I run out of data. T-Mobile, unlike PTEL, rounds data useage UP to the next megabyte!