Correcting "no hangup" or "hook on" in 3Com / US Robotics Sportster 5686?

The 3Com / US Robotics Sportster models 0839-08, 0839-09, 5686-01, 56886-02, 5686-03 and 5686-04 appear to have nearly identical component layouts.

Although the modem can accept commands through the serial port and dial out, it cannot standby in "hangup" or "hook on" mode. The hook on mode provides the physical equivalent of a handset returned to the telephone cradle. This indicates that the direct access arrangement (daa) has some componenet damage.

In older modem designs, the daa requires replacement of the component that provides a voltage decrease such as closing a relay or a pull up resistor. What component provides this electrical equivalent in these particular modem circuits? Is this component obtainable by the average hobbiest?