I could've sworn there were a few who managed to get BOTH on T-mobile, (theres a whole thread about it after denim), and a few people said they got them working on other networks, with a few issue like no SMS on 3g, and stuff, but then the thread just died... I'm gonna go look for it
Why not try Straight Talk with the ICON. It uses Verizon signal and gets LTE, and MMS with no problem. Cheaper than AT&T as well. I used it for a while and had no problems with it at all.