PURPOSE: To establish the system such that number of trunk lines is saved between TDMs, the line efficiency is improved and the telephone charge is saved between INS net stations and public line networks of INS and PSTN are interfaced.

CONSTITUTION: Master equipments 6,7 each comprising a PBX 20 and a TDM 21 are connected to a digital public line network INS 4 and an analog public line network PSTN 5. When a telephone set 1 makes dialing of a PSTN number of a telephone set 2, the PBX 20 selects a trunk for the TDM based on the PSTN number and gives the signal to the TDM 21. The PSTN number is set to a subaddress and the TDM 21 makes dialing to the master equipment 7 by using the INS number set in advance to the trunk. Band compression is implemented for a signal sent between the TDMs of both the master equipments. Seven channels are allocated to 2B channels (128k) of the INS by implementing 16kbps voice compression. The master equipment 7 extracts the PSTN number from the sent subaddress and makes dialing to the PSTN.