This document describes one reason why the analog phones attached to the VG224 Voice Gateway, which registers to Cisco CallManager as the Skinny Client Control Protocol (SCCP) gateway, receive no dial tone from the VG224 Voice Gateway when they are off-hook and provides a solution.
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The analog telephone attached to the VG224 Voice Gateway receives no dial tone when the phone goes off-hook.
Figure 1 shows the topology of this problem.
This problem is a configuration issue. It is related to idle battery voltage. Foreign Exchange Stations (FXS) need to supply DC battery and AC ringing to enable the connected telephone equipment to transmit speech energy and to power the ringing device of the telephone equipment.
The show tech-support command output shows that the FXS idle voltage is set to low (see the arrow A in Figure 2). This is the root cause of the problem.
Figure 2—Partial Output of the show tech-support Command
The solution is to set the FXS idle voltage to high (see arrow A in Figure 3) when you configure each individual voice port on the VG224. This changes the idle voltage from -24 volts (low) to -48 volts (high).
Figure 3—VG224 Sample Voice Port Configuration