Comcast bills....they are the worst.
From my experience, here is what I know:
Verizon iphones are unlocked, period. Last year I bought my 5s from verizon, and have since then used it on att and tmobile.
Att will unlock your phone if you have paid it off (not sure how long you need to use it with them first, probably 30-40 days). This was verified with my current phone, an lg g3, that i am now using on tmobile.
A tmobile iphone purchased from tmobile is locked. Why? Because they have the jump plan, where you pay monthly until you pay the phone off. Once you have paid the phone off, assuming you have used it on tmobile for a minimum of 40 days, they will unlock it. Period. So if you buy an iphone from tmobile on launch day...you need to have a tmobile account.
If you buy a tmobile iphone from apple, it is unlocked. It will work on tmobile, att, and other smaller carriers. It will NOT work on verizon or sprint. Why? Because verizon and sprint have a ridiculous policy where the phones you use have to have been purchased from, or designated for their network.