There could be many reasons for your not being able to make a call. Have you checked your minutes and or if there is a restriction on the phone (through your provider).
Also, is it all the time that you can't make outbound calls, or only at certain times. Say only when on another call, in a specific app, on the net, etc? If it is when doing one of these, but then it works when you dial out when not in anything else, then I would look into if the app, etc., is restricting the calls.
Next, did the problem start after a new app installation or update? If so, try uninstalling the app or update and see if the problem is resolved.
If none of these work, then I would suggest contacting your provider, the manufacturer or the place where you bought it, as it may be a defect.