An office-line trunk circuit is provided to selectively operate on ground-start and loop-start modes. In this circuit, a DC current detection circuit has (i) a first photo coupler detecting a DC loop current caused responsively to supply of the ground to a tip line and a negative power to a ring line from an office bilateral trunk and (ii) a resistor whose resistance is adjusted to prevent a loop start in the office bilateral trunk and allow DC current to be detected. The first photo coupler detects the loop current using the resistor. An incoming detection circuit has a second photo coupler detecting a ring signal coming from the office bilateral trunk. A DC sink circuit has a diode bridge placed between the tip and ring lines and configured to have a DC circuit provided with a transistor controlling the DC loop current so that the office bilateral trunk is allowed to selectively be loop-activated and loop-held.