Gary, the IMEI code is normally underneath the battery of your devide and additionally on the box where it was stored when you’ve bought it.
The IMEI registration it’s related with the possibility to address a phone call to a specific apparatus/person/identity.
When you do initiate a call, within millisecond occur some process related with the “origination” of your call that will in one hand address your call to the desired recipient and on the mean time allow the carriers to address the charge for the phone call.
MNO’s are losing billions a year in the so called “IP gray area” therefore are trying to fil this gap by implementing the capabilities to addres a charge.
Without mentioning eventual security related issues.