Calls going to partner’s phone

Before I got my iPhone XR, I was using an Android. I accidentally shut it in a car door, so my boyfriend said I could use his old iPhone 6s until I could go grab my current phone.

I popped my SIM into his old phone and it ended up giving me all his photos, contacts, and apps on my new phone once I moved it over.

I am almost certain that is what connected our phones. Every time I get a call, it will ring on mine and, if I don’t answer, it will ring on his and he gets my voicemails.

The only advice offered online is to be sure we aren’t sharing an Apple ID (we aren’t.)

I have considered getting a new SIM card if possible but I wanted to keep my number.

He also has an iPhone XR if that helps.