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New Member

Modem -> FXS -> Callmanager 4.1.3 -> PRI problems

I am having serious problems with the following setup:

Modem -> FXS (2821, MGCP) -> Callmanager 4.1.3 -> PRI. I can't get any of my modems to connect reliably or with good speed. I have looked all over for documentation on Cisco, but so far have found very little.

I am looking for real-world solutions that have worked for others in getting this scenario working properly.

I have tried the following with no luck:


mgcp modem passthrough voip mode nse

mgcp modem passthrough voip redundancy

mgcp modem relay voip sprt retries 6

mgcp modem relay voip mode nse



no echo-cancel enable

no non-linear

timeouts call-disconnect 15


voice cut-through alert

fax rate fax

supplementary-service pass-through

Thanks in advance,



Re: Modem -> FXS -> Callmanager 4.1.3 -> PRI problems

command "mgcp modem passthru" to enable the gateway to send and receive modem and fax data, should able to support your requirement.


New Member

Re: Modem -> FXS -> Callmanager 4.1.3 -> PRI problems

This is what we use for faxes and modems for our mgcp gateways with ISDN PRIs.

Faxes work fine (granted they are not the new high speed...dsp have hard time keeping up with the input feed from the higher speed faxes, will fail the fax).

As for the modem, the config will detect the modem signal and change the codec to g726r32. This works, but will intermittantly drop, sometime frequently, and/some times get junk characters sent (ex: using modem for procomm to connect to remote devices). The modem transmission might work better bumping this up to g711, but we don't want to use that much bandwidth, so half the time we end up dialing via PSTN, by passing modem VoIP dail-up.

working config - output from 3745:


mgcp call-agent 2427 service-type mgcp version 0.1

mgcp dtmf-relay voip codec all mode out-of-band

mgcp rtp unreachable timeout 1000 action notify

mgcp modem passthrough voip mode nse

mgcp modem passthrough voip redundancy

mgcp codec g726r32 packetization-period 10

mgcp package-capability rtp-package

no mgcp package-capability res-package

mgcp package-capability sst-package

no mgcp package-capability fxr-package

mgcp package-capability pre-package

no mgcp timer receive-rtcp

mgcp timer nse-response t38 500

mgcp sdp simple

mgcp fax t38 inhibit

mgcp fax t38 gateway force

mgcp rtp payload-type g726r16 static

mgcp rtp payload-type clear-channel static


mgcp profile default