How to get proof I have changed my phone due to previous phones problems

Hello!

In 2019 my iPhone XS started to have problems with bluetooth and I went to BCCS office in Latvia. They found out that it is impossible to fix this problem and send my old phone away to gave me a new one.

I got a new phone it works great but the problem is that my previous phone which was sent away was not my but it was mobile companies phone (I was paying them for using this phone) and when I came to give them iPhone XS back they told me that phone numbers don't match (it is not the phone they gave me) and they can't accept this new iPhone. I was contacting BCCS office and I was asking for any document which can proof that they took my old phone and gave me a new one but they said that came too late and they don't have this paper anymore and they suggested me to contact apple support. They told me that apple can see in the system that phone was changed and can send a document which proves this fact

I hope they are not fooling me...

I hope you can help me somehow

Have a good day,

Elina