Almost all a/c´s have two sets of interphone jacks,one for flightdeck communications and one for service communications.
The flightdeck interphone jack´s usually situated inside the groundpower connection panel or(on widebodies and others)on the nose landing gear,adjacent to the apu fire extinguisher buttons and nosegear lockout pin assy.
This jack is used by the engineer/startleader to communicate with the flightdeck during push back/startup procedures,as well as other technical issues.
The service jack provides direct contact with the cabincrew at their designated stations throughout the cabin.
Service jacks can also be found on certain parts of the fuselage,close to the engines,wheelwells and such..these are placed there for maintenace reasons only,to enable contact between the engineer´s on the ground and on the flightdeck.
They´re only used during routine/sheduled maintenance checks/repairs.