Deregister iMessage
Bulgaria +359,is not on the list.
iPhone 5, iOS 10
If you are following the steps in this article and having that problem, contact Apple ID Support.
iMessage on your iPhone or online - Deregister
Apple Support by e-mail or chat Work through the screens to get a chat option.
Or fill out and submit this form. The form is for iTunes, but it will get passed to the ID Support section.