Are you crazy to call back an unknown number? Do you know someone to call from that country code? Have you heard about sim cloning?

If they want to talk to you they will call again. It could be a trap. There are lots of mafia who are doing cloning of sim cards and using it for illegal purposes. If you call back your sim could get cloned and will be used by them for illegal activities which could lead you to great trouble. If you are getting missed calls from unknow numbers just ignore it for being safe.