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New Member

VOIP with pstn & PBX

We have a VOIP setup that connect 2 PBX s by IP network.

I want to know how could I initiate a call from branch office extention, connect to the head office PBX and from that acces PSTN around the head office.


New Member

Re: VOIP with pstn & PBX

The information provided is not enough to answer this queery. Please specify what cisco equipment you used to connect both the offices.

The typical call flow you are looking may be like this

1. Call is initiated from your branch office phone which goes into your PBX directly.

2. As your PBX is connected via the IP n/w to your head office, The call is converted into IP and sent to the Head Office.

3. At the Head office the call should be recieved by Cisco IP equipment and will be terminated through that gateway into the terminating PBX which will terminate the call to the end user.

I can further answer your queries if you can let me know the details of the equipment you are using.


New Member

Re: VOIP with pstn & PBX

Hello Kiran,

Our network is exactly wht you explained.

1.Branch office router 1751v with FXS (seem to be a mistake because of not using FXO but I cant understand disadvantages)

2.Head office routers 3662 with NM-HDV-2E1-60

3.Lucent PBX


Re: VOIP with pstn & PBX

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.

New Member

Re: VOIP with pstn & PBX

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).

phone<-->PBX<-->Router ===WAN===Router<-->PBX<-->phone

The Remote office PBX will need to be configured to route all calls for the Head office to the Remote office Router via the FX interface connection.

The Remote office Router will need to be configured with a POTS dial-peer (remote office pbx) and a VoIP dial-peer (head office router). Check out Cisco docs on configuring dial-peers.

This same thing will need to be accomplished in reverse on the Head office PBX and router.

New Member

Re: VOIP with pstn & PBX

How are your pbxs connected to the routers? fxo, e&m, or isdn? The type of connection will determine how you will place the call.


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