Calling from Mumbai MTNL Landline to Mobile Numbers outside Maharshtra


i am trying to call some mobile numbers from my MTNL landline in mumbai. the numbers are based outside of mumbai ie in states other than maharashtra. when i dial the full ten digit number, it gives me BEEP BEEP sound for some time and then it says - pls check the code u are dialing. what am i dong wrong????? and is it not possible to dial mobile numbers direct from landline to other states. do i need to have STD activated in this case. Please Help.