I am from Pakistan and I am unable to deregister my old phone number online as there is no option for pakistan in the drop down code list of countries.

Hi kamran161,

It sounds like you want to deregister iMessage, and you're running into an issue. We'd love to get you some help with that.

This page has some great information, and it goes over the steps to deregister iMessage:

Deregister iMessage on your iPhone or online

If you're using the steps in the above link, and you're unable to proceed with deregistering, please contact our support team for further assistance. You can reach them here:

Get Support

Have a nice day!