When trying to make calls and your call is rejected; it might because of any one of these reasons:
1. Interference, especially in areas with direct obstacles to base stations e.g buildings, trees
2. Jamming, especially if you are in a national security protected area (think state house) there might be deliberate signal jamming there
3. Network congestion, you can't connect on one goal if the network is congested especially nowadays when there is a lot of these promotions going on... "Dhora a day!"
4. Outgoing call firewall, you might have set it by mistake just check your settings. Otherwise calls won't get through
5. No airtime, this might sound trivial but if you don't have any airtime your calls will definitely be blocked by the network.