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New Member

1751 connection to PBX for VoIP - just a simple HOWTO.


Does anyone have any pointers for configuring a 1751 BRI connected to a Siemens PBX?

I have a 2/FXS VIC and a VIC-2BRI in 2 x 1751 routers connected via a serial line.

Phones plugged into each router on the FXS port have no problem communicating, also over the serial connection, however I am at a slight loss as to how I should configure the VIC-BRI port to enable me to dial from the FXS ports into the PABX.

The connection between the router and the PBX is fine, q921 and q931 signalling checks out.

Serial line is using "ip unnumbered loopback 0".

Sorry, not much experience in VoIP setup - any help much appreciated.

I have searched, but have not found what I am looking for.

Also, the "isdn incoming-voice voice" command which is often refered to does not appear on my IOS {12.2(8)T1} (not even hidden)

Any additional info on the Siemens side I should know? PBX has been set up as Master.

Right now I am able to call the phones on the FXS ports from the PABX, but I cannot call the other way (into the PABX from the FXS ports). I get a

"Mandatory information element missing" from the ISDN debug.



Re: 1751 connection to PBX for VoIP - just a simple HOWTO.

Check with the following debugs for identifying the problem ?

'debug vpm all' 'debug isdn q931' and 'debug voip ccapi in'

Did you check at the PBX if it is fine?

New Member

Re: 1751 connection to PBX for VoIP - just a simple HOWTO.

Sounds like the router don't know where to router the calls. You need to tell the gatekeeper where to dial to. Need to set up a dial-peer as pots. On this page : , look for Scenario 1, and GWY-A1. It hows the dial-peer and how it points to the S interface. See the destination-pattern. Also, are you running native or h.323 SIP?? Now I don't know ISDN and could be sending to the wrong way. But the doc's look like that.

Good luck


New Member

Re: 1751 connection to PBX for VoIP - just a simple HOWTO.


Probably you already have solved your problem by this time.

I had the same problems connecting a Hicom 150 with a 1751 router. The problem got solved when I upgraded to 12.3a. It did not work on 12.2(8) (The command does not exist, it's a confirmed bug) and did not work on 12.2(13)T.

I used the ISDN BRI interface on the router as Network Side and configured the Hicom 150 with ECMA-QSIG Slave PRI (cr-length).

It's now working fine. I expend two months on tryings and traveling.

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