I've had the number call on both my phones. 

Don't call it back - you will get charged and they will get a commission.  This stuff is really common. 

There was a similiar one like this done from Russia.  When you returned the call you were charged a fortune and the person you were calling was making a huge amount from you.

If you don't know the number, or the country code, don't ever return these sorts of missed calls.