Analog and T1 trunks are incapable of delivering caller ID information to the public network. However the serving Central Office (CO) has the capability to generate a calling party number that has been assigned to the trunk group on which the call originates. The calling party number generated by the public network is known as Automatic Number Identification (ANI).

In order to block or unblock the generation of an ANI number by the CO, the Caller ID Block/Unblock feature can optionally send - based on a new trunk attribute and based on the existing trunk dialing method - the standard caller ID blocking or unblocking access codes as a prefix to the destination address digits.