Lost my second iPhone in Italy

I have 2 iPhones. 1 usually have my Australian phone acct chip. In the phone I’m using. The other older phone had a My3 chip from UK. I swapped the chips because my for travel to italy. I took both phones. I popped my Australian acct chip in the old phone but now it’s missing, the phone.

I have the other iPhone with My3 acct chip with me. When I logged in find my phone it registered the one I’m using even though I put the Australian acct number up. The one I’m using is not using that number. Does the serial number of a phone register it for “find my phone app?” Or is it the phone number?