Will switching device from virgin to st use sprint towers and will it lock device on st?

[AC Question]

I would like to try Straight Talk on my existing phone, ZTE AWE. It seems that the phone will work with the BYOP feature. I really wanted to try the phone I have been using on Verizon towers. I have been told (at Walmart) that if I use a Virgin phone, it will still use the Sprint towers. I read that activating a BYOP phone on Straight Talk will lock it so that it cannot go back to Virgin. Can anyone explain this to me? Thanks