General Can't make calls

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http://androidforums.com – Dear all, please help a relative newbie in desperate need of help! I have an S2 4G LTE that originally came from S. Korea but hasn't been unlocked, only updated to 4.0.4 about 6 weeks ago and has been working fine. 2 days ago I stopped being able to make any calls. I can receive calls, send and receive SMS and all mobile internet is working fine. I honestly don't know if it is a carrier or device problem and would be eternally grateful for any help. P.S I'm currently in Ghana and using the phone on MTN if that is of any help. (General)