i don't believe the program for ATT phones on straight talk is the same as verizon phones on straight talk.
from what i understand from researching this a few months ago was that only verizon 3g phones could be ported to ST. if it was a 4g phone then ST would not allow you to use on their network (more likely verizon won't let ST on their 4g network and are just leasing them the less used 3g network). The only way to use a verizon 3g phone was to pay for ST to unlock it.
i highly doubt you can take a 4g verizon phone and use it on ST, if you can't find the information you need on the web then call/email ST and ask them. the few times i had to email them they got back to me within a few hours.
---------- Post added at 01:03 PM ---------- Previous post was at 12:55 PM ----------
i see you are wanting to use a ST ATT sim in the verizon one, to use on the ST network which would use ATT towers.
i'm not sure if this is possible. i would ask ST and see what they say. it could be that they say NO just because the person you get doesn't understand what you are trying to do. Perhaps the other way would be to simply try it and see what happens... but if you are currently using the phone on Verizon, then this would probably end your contract and unlimited data.
however, i still stand by my conviction that you cannot use verizon 4g devices on ST where ST uses Verizon towers.