Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member


We are planning on a rolling out a customer-oriented, CPE-based, on-net VoIP system, and are running into some problems.

We have the following design needs:

1. Deliver customer Internet via T1, MOE, etc (done)

2. Deliver reliable VoIP service (done)

3. Be not super expensive (why we're rolling Cisco+Asterisk)

4. Deliver easy, reliable modem & fax support via Cisco modem passthrough/relay

- we have noted that T.38 is a disaster, and faxing at even 9600baud over VoIP is unreliable.

- We are trying to avoid rolling a POTS line to Customer's site.

That last #4 is the hangup.


Our design (attached) is to plumb Local Toll + Fax/Modem stuff out of an AS5300/PRI/PSTN, and all LongDistance/Overflow Local calls across Asterisk+SIPCarrier.

Our CPE device, in our present implementation, is a Cisco IAD2431 - lets us deliver a T1/etc of Internet, VoIP and standard FXS in one box - all across SIP. We are currently able to deliver DIA, standard VoIP & FXS/PSTN without a problem, but the fax/credit card/alarm system is hard.

Our plan to resolve that is to use Modem passthrough/relay and deliver the Fax/Modem service directly to the PSTN on a PRI, rather than trying to monkey with T.38/FoIP/Our SIP provider.

The problem is that we can't seem to make the IAD detect a fax call and send it to the AS5300 rather than Asterisk. We -can- PLAR an FXS port to the AS5300, but this is a pretty clunky solution. Detecting the fax call and routing it across the passthrough/relay-enabled service is critical to our problem-free fax/modem needs.


We have two possible solutions - attach the IAD directly to Asterisk (may not work), OR plumb -all- calls through the AS5300 ($$$):

1. customer -> IAD -> Asterisk -> AS5300 -> PRI/PSTN

- Our concern is that Asterisk may munge the Cisco-specific SIP notification of modem passthrough/relay stuff

2. Customer -> IAD -> AS5300 -> (PRI/PSTN)|| (Asterisk-->SIP)

- Concern here is that it all calls have to be proxied/registered with the AS5300, and we would like to avoid that ($$$)


1) Anybody know if Asterisk can proxy a Cisco modem passthrough SIP session without problems?

2) Can an AS5300 match fax/modem traffic and drop terminate it on the PRI automatically?

3) Is the AS5XX0 the best device for this in the sub-$10,000 range?

4) Do you have other suggestions concerning how to accomplish this?




Re: VoIP+Fax+Asterisk=Howto?

Hi Randal,

I don't think you're going to find a solution that lets you 'policy route' based on the call being fax or not. And if so, it will not be reliable enough.

The best way to configure this is to make sure that you're using a fax method that is interoperable with both sides.

The options would be:

voice service voip

fax protocol pass-through g711ulaw

fax protocol t38 fallback none

Modem relay isn't a suggested configuration unless you're in a pinch for bandwidth, and the other faxing methods include the use of NSEs which are Cisco proprietary.

It should be noted that for an outgoing fax call, the router responsible for 'detecting' the fax is the terminating gateway. This means if the customer is placing outgoing calls, it would be on the 5400 or Asterisk. If you're having trouble, you may want to try 12.4(15)T8 as there were a few issues with fax detection.



New Member

Re: VoIP+Fax+Asterisk=Howto?


Yep, we determined that we can only detect the fax call at the terminating or answering end, so having our Asterisk SIP-proxy won't do any good -- can't listen in on the call, and then switch to the PRI/PSTN service mid-call.

That said, we have determined that the best way to do it is to PLAR the extension OR to have a prepended string on the ANI that we can match on and call route with before the call is connected.

Thanks for your help!

New Member

Re: VoIP+Fax+Asterisk=Howto?


I hope you had some luck with routing calls using different dial peers (that is probably the best way to tackle the problem) and routing fax traffic using the network diagram that you had mentioned.

Adtran TA products support SABR routing which let you do the same.

BTW, are you using T.38 on the call leg for fax (before you terminate to PSTN)? If so, would you mind sharing if you were successful with that?

CreatePlease to create content