Blacklisting device??

So I have a ongoing contract with Telkom Mobile. I lost the mobile device and immediately suspended my number so no one could use my sim. The phone is insured and to claim a new one I need to provide my insurance with the Case Number from police as well as ITC number that phone has been blocked/Blacklisted and the device cant be used by anyone else.

I went to Telkom mobile and was told that they cant issue a ITC number or blacklist the device but need to contact their technical team. I have the police case number and the device details (even the box with original slip received by Telkom). So I went on the online chat and chatted to one of their agents requested device to be blocked and supp**** all my information as required.

The next day I got a phone call from somebody at Tellkom telling me that they cant blacklist my device. Someone picked up my device, took my sim card out and entered their Cell C sim card and is now using my phone. Telkom just said there is nothing they can do. I (who am a telkom client) needs to contact Cell C and tell them to blacklist the device...???

Isn't this then the reason why you want to blacklist a device in the first place? to stop somebody else from using the device?

Anyway I contacted Cell C and the lady I spoke to actually laughed at how ridiculous Telkom is and said she cant do anything obviously. So back to telkom. Had another few requests to have the number blocked and just straight up that they cant help me. They cant even give me a decent explanation to why not? It is my device, the IMEI number can be used to verify the device so whats the problem? I am so enraged by this useless no-service provider. As soon as my contract expires I moving to someone else.