Main apne local network. Extention se call krna chata hoon. For example mera office ha os main aik land line number hy. Office mian 5 room hyn hr room ki alag extention hy. Kisi b room k set se 9 press kr k mn local call b kr sakon. Or enternet ki madad se aik do extention dosri city mn b use kr sakoon. Es k lye muje cisco ki kon c hub ya router chahye. Or username or password b ho jisko mobile se kehen se b use kr sakoon. Es se muje aik se dosre city mn free mn call krne ko mil sake gi.