This read-create Integer32, ranging 1 to 65535, identifies the DS1 line for this endpoint, generally corresponding to the ifIndex of the physical interface terminating that line, and per the description cannot be changed once the row is active. An admin sets this during endpoint provisioning to bind the logical endpoint to a specific physical DS1 interface on the gateway. Because it's locked after activation, an admin who needs to move an endpoint to a different physical line must delete and recreate the row rather than edit this value in place.