I've seen similar scenarios to this. the PBX is FXO I believe, and when router has FXS the PBX simply brings that line offhook. The voice-port in the situations I've seen had connection-plar that automatically called a # at head end which went to PBX.
For users at remote site, when they placed call that went over the IP network they got dial tone from head end PBX and entered a # for that system, etc.
Similar setup in reverse from head-end with the customers i've seen. In the head end PBX they had certain #s associated with each remote site and when dialed 111 it would bring port 1 offhook on PBX which was associated with timeslot 1 and on the router the ds0-group for timeslot 1 voice-port had connection plar for # for that particular remote site.
This is just one of many several ways to configure it...
Also, on the 3660 with the NM-HDV there are several bugs with it so use the latest IOS. 12.2(6a) mainline fixed a DSP Timeout problem but there have been fixes since then in both 12.2 mainline and 12.2T. Current versions are 12.2.13 for mainline and 12.2(13)T1 for the T train.
An FXS interface provides a line voltage to the connected device. Normally a Cisco FXS interface would be connected to an FXO device (phone). The phone would use that line voltage as its power source. You need to determine if the PBX is providing line voltage for the connection to the router. I bet it is.
The physical connectivity as I understand it is; Branch router connected to PBX via 1 FXO interface (1 call only), Head office router connected to PBX via HDV-2E1 (up to 60 calls).
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