There are two types of call recording for TDM trunks: Passive and Active Recording.
Passive Recording is completely transparent to a telephone system. A passive integration involves the recorder passively sniffing (monitoring) a network for audio and call information to copy to the recorder. There is no impact to the telephone system at all.
Active Recording involves a switch-level integration that requires additional data from a CTI application from the PBX or a third-party vendor. This additional data facilitates an extended set of call controls, delivering greater flexibility and functionality as a result of the interaction between the call recorder and the PBX.