Deregister from iMessage
I want to deregister from iMessages, but i can not do that because Serbia
(+381) dont exist on the list
User profile for user:
Community User
You are right, Serbia is not listed here:
Deregister and Turn Off iMessage - Apple Support
If you still have access to your iPhone, you can deactivate iMessage yourself from there, in Settings -> Messages.
If you no longer have your device, you should contact Apple Support directly, I'm sure they can help deregister the phone number.
Contact Apple Support:
Contact Apple for support and service - Apple Support