How to deregister iPhone number when numbers out of service

I had a business device and used a personal iCloud on it. When I gave employer the phone back he sent me a message to my new personal cellphone claiming I needed to provide a pin to deregister iMessage and iCloud on business device. However after giving the code to him a message appeared and said phone number has been registered but it was my new personal device phone he deregistered, and he actually retired the personal phone to his business. Ever since I have issues with my service, I can’t send iMessages and I can’t deregister the number as he claims he disconnected the sim. But I believe sim is still on and he is tracking me still! How do I deregister that old employer phone number from my current device? I got a new sim and it didn’t change anything. And I can’t remove any MDM on personal phone as it doesn’t show the configuration profile. Help it’s been a year now!