I can't add my mobile number, because the carrier isn't listed in the device list.
Do you mean getting the "Verified Account" badge on your profile? If you do then you can't - it is closed to the public
With regards to the mobile verification - if your carrier is not available then read this article http://support.twitter.com/articles/41611-which-mobile-carriers-does-twitter-support
It means that you will probably be able to send texts to Twitter but not receive any.