How does the carrier even know what phone you are going to use? My last SIM was bought in store at T Mobile, for cash, and I can freely move between phones. I never buy phones from carriers. My Pixel as bought directly from Google and immediately flashed with Graphene.

IMEI maybe?

This could have also been an issue where I needed to activate on some other phone before transferring to a Nexus 5X in order for me to get it to work.

Verizon is known to be the most restrictive carrier in the US as far as allowed devices.

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.