Ending a call
I am unable to end a phone call on my iPhone 11. The button remains green and the call stays connected. Why won’t it hang up?
Windows, Windows 6
If it were me, the first thing I'd do would be to contact my carrier. You may have to be a bit persistent. Even if you select the options for "tech support", the first person you'll get will be a customer service rep with a handbook and a bit more training. If they can't help you, politely, but firmly, ask for the next level of support.
You may also want to restore your phone.
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support