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PSTN BRI Voice routing via FXS to PBX

We use a 3640/IOS 12.3 for BRI data and analog V.90 serving. I want to migate the PSTN ISDN to also route voice calls via the FXS cards to our PBX via pots. The dial-peer config is unclear as this is not true voip. Do we use vofr, ras, what? I want an inbound voice call from the PSTN BRI indentified using SPID2's LDN to ring a FXS port direcly (plar?). And, if an FXS port goes off hook, I want to dial out a ISDN BRI port to the PTSN. What protocol? H323, vofr. Does the route have to be processed internally as a voip call or what? I can route a pots dial-peer from one fxs port to the other, it works, but I can't declare a BRI port on the BRI8B-U as pots. Wish I could. Any suggestions would greatly appreciated.



Re: PSTN BRI Voice routing via FXS to PBX

Yes, you need to use VOFR.

New Member

Re: PSTN BRI Voice routing via FXS to PBX

Thanks for your suggestion. Can you be more specific? If I declare a DCLI for the vofr dial-peer, then what's the mate side of the vofr dial-peer? My config is attached in a reply further down the thread. Can you gice me an example of the dial-peers for the vofr bri & the pots fxs?

Thanks in advance,


Re: PSTN BRI Voice routing via FXS to PBX

Hi Bob,

I'm in the process of labbing this up out of my own morbid curiosity, 'cause a) I'm a glutton for punishment ;-) and b) it seems to me that you should be able to declare the bri a pots port. If I come up with anything there, I'll certainly let you know.

Normally, your config would consist of pots dial-peers that point to the bri and to the fxs, with incoming-called and destination-patterns set according to your dial plan. The call is tdm switch within the router. No voip (h323, mgcp, etc) here. Also, no need to hairpin via some loopback on a frame-relay link (vofr). And there should not be a need to use callmanager or other call-agent to route this call.

Just a simple tdm hairpin; pots dial-peer to pots dial-peer.

Now, another possibility is that the bri8b-u doesnt' support voice. If that's the case, you should consider the newer vic/bri cards. Here's a couple of links to look through.

Understanding Voice BRI cards:

Configuring Voice BRI cards:

Last possibility is that it is an IOS issue. Can you post a show diag and show ver?


Re: PSTN BRI Voice routing via FXS to PBX

I couldn't find a spare 1bri-u. All I had handy is a wic-1b-u. Older card - yours was the next generation, but I'm not sure it'll matter much since the application they were designed for is pretty much the same.

Anyway, just tried a few scenarios for you:

1) BRI in a NM-HDV

2) BRI in a NM-2V

3) BRI in a NM-2W

4) BRI in a 2651XM WIC slot

I could "see" the card in both 3 & 4, and could not see it in 1 & 2.

In 3 & 4, as you indicated, the parser refused to provide "voice-port" configuration for it.

I tried IP Plus/voice code in 12.3 & 12.3T flavors, but to no avail.

Looks to me like the config you want is not going to be supported.

New Member

Re: PSTN BRI Voice routing via FXS to PBX


Yep, i would have thought I could route the bri as a pots dial-peer to an fxs dial-peer as it does work between 2 of my fxs's. However, the bri port is disallowed in the dial-peer creation as a pots dial-peer. I sure this is because the bri doesn't show as a voice port. I can define the bri port as "isdn incoming-voice voice" vs. "isdn incoming-voice modem". This difference keeps the modems from answering the call, so I get no answer on an inbound ISDN voice call.

Since the router handles the TDM from the BRI for the modems, I just can't figure how to route the TDM to the FXS card within the voice module. I keep telling myself I'm overlooking something, however, I can't fing an example config anywhere. Maybe it's just not as cool as all the voip stuff.

The show ver & show diag are attached:



Re: PSTN BRI Voice routing via FXS to PBX

Sorry I wasn't more helpful. FWIW, at least you're not going and buying new hardware for this 3640.

I think what it comes down to is that the older wics can't leverage dsp resources for voice. And IOS won't treat the card as a voice card without the right voice 'hooks.'

Definitely seems counter-intuitive.

Can anyone from Cisco address this question? Why do we need dsp resources to tdm switch voice?!?!

Re: PSTN BRI Voice routing via FXS to PBX


I haven't had the chance to test in the lab, but have you tried something like the following:

Let me know if you get the chance to try it.

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