Comment by skirmish

They do know, I experienced it myself.

I had a testing phone and my regular phone, I kept swapping the same Verizon SIM between them. It worked for maybe 3 swaps, then Verizon blocked my account and sent me an email: suspicious activity detected, the SIM is blocked and cannot be restored, the contract has been terminated, you will receive the refund in mail.

Good riddance, US Mobile is so much cheaper and pleasant to use. I am scared to swap SIMs with them now though.