Recording units connected to handset wire

Some commercial telephone recording devices are connected between telephone base part and handset. Those devices are small boxed which are placed between telephone ant the modular wire coming from telephone base part to handset. The telephone conversation can be taken from that wire, because it is carrying the microphone and speaker signas which are connected to base part of the telephone. The signals going in this wire between base part and handset are no fully standardized, which means problems to those who are trying to make devices which connect to this interface. But this interface has the advantage that it can be also used with those digital telephones, where recording from line wires is not possible.
Typically the handset of the telephone is connected to the base of the telephone using 4 wire connector. Two of those wires carry the signals to the handset spaker and two other wires on the other side of the connector carry the microphone signals. The standard handser wiring used in telephone in USA has Green and White leads connected to earpiece and Black and Red wires are connected to microphone.
There are differences on the signal levels used by deferent telephones, so typically the equipment which connect to handset connector have some adjustments to make them suit to many telephone types. Well adjusted recording interface which is connected to handset interface gives good balance between the local voice and voice from distant end (much better than recording devices which connect to telephone line).