... ,electrostatic plastic sprayed surface. good impact resistance.2.2 The telephone adopt stainless steel keypad, which contact resistance≤30ohm,life cycle ... random . it can add or decrease anytime without restriction . Each telephone is the independent workstation, will break down not to interfere ... amplification call directly or after sevral ring ( 1 to 7 voice can be set ) ,or more ring (it can be ...