Hi Michael and welcome to the forum.
This is a Customer to customer forum so you will need to continue with customer services of Vodaphone.
Can I ask where you purchased the iPhone from ? Was it directly from Vodaphone.
Inserting another SIM card in the iPhone does not affect its imei number and would be able to check its history via its serial and imei number so Id go with that information.
Make a few chargeable calls with your sim in the phone and contact them again as is helps to re establish your imei for the phone back onto your Vodaphone account.
Will you let us know how you get on.
😎
Current Phone >
Samsung Galaxy s²⁴ Ultra 512gb.