Cannot Deregister Phone Number due to region

After trying to add my phone number to imessage i was told that the phone number is already in use with another account. I cannot access this account because in order to verify my identity apple needs to send a text to an older phone number which i currently do not have. After doing research I was told i can simply deregister my phone number but the website does not allow me to because the region (Kenya) is not there. What can I do?

Thank you