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dial up modem problem (wont train) on FXS

Christian Isla
Level 1
Level 1

The scenario is that the UC has a PRI for voice services.  I have a dialup modem on my FXS port which is trying to place a call.  The far end answers and locally I can hear both the receivers tone and my own shhhhhhh local sound.  However, on the far end I hear the receiving tone (it's own) but I only hear the shhhhhh from the caller (behind the FXS) for a split second.

both modems are setup to keep audio up for the entire duration so i can hear what is happening.

Why is the UC blocking the shhhhhhh audio from the FXS to the receiver?  I've disabled ECAN/NLP and CN  but still no luck.

Also, if the call is generated from the PSTN to the FXS the problem is the same but reversed.  The caller tone is heard in both locations but the sshhhhhhhh doesn't make it to the FXS modem (receiver) this time.

I'm running UC520 -  Version 15.0(1)XA3a,

some relevant config:

card type t1 0 2
network-clock-participate wic 2
network-clock-select 1 T1 0/2/0

trunk group ALL_T1E1
hunt-scheme longest-idle
translation-profile outgoing PROFILE_ALL_T1E1
voice class codec 1
codec preference 1 g711ulaw
codec preference 2 g729r8

controller T1 0/2/0
cablelength short 133
pri-group timeslots 1-22,24

interface Serial0/2/0:23
no ip address
encapsulation hdlc
isdn switch-type primary-ni
isdn incoming-voice voice
isdn supp-service tbct
isdn supp-service name calling
isdn sending-complete
trunk-group ALL_T1E1
no cdp enable

voice-port 0/0/0
input gain 3
no echo-cancel enable
no non-linear
no comfort-noise
timeouts ringing infinity
bearer-cap 3100Hz

voice-port 0/2/0:23
bearer-cap Speech

dial-peer voice 62 pots
trunkgroup ALL_T1E1
corlist outgoing call-local
description **CCA*North American-10-Digit*10-Digit Local**
translation-profile outgoing OUTGOING_TRANSLATION_PROFILE
preference 3
destination-pattern 9[2-9]..[2-9]......
forward-digits all
no sip-register

dial-peer voice 100 pots
description ** incoming dial peer **
incoming called-number .%
port 0/2/0:23

1 Accepted Solution

Accepted Solutions

STCAPP seems to interfere with modem audio modulation for some reason. Make sure you set it up as either "common-Area Phone" or "Fax". You'll end up with a destination-pattern in a dial-peer for pots. This will also remove the ephone thus the need for a license.

View solution in original post

9 Replies 9

Level 4
Level 4

Is this a hybrid config or is this all CCA?

It's pretty much a fresh box used to test PRI.  CCA was used to activate the PRI card and inboud outbound calling.  Nothing else was enabled.  There are only 2 IP phones connected (this is just for testing purposes).

Very little CLI.

1.     CLI was used to change some custome translation profiles.

2.     added a custom dial-peer (not used in these calls).

3.     disable ECAN/NLP and CN

4.     create another vlan (used for managment)

Calling using an analogue phone, it seems to sound ok in both directions... But if I recall correctly sometimes it sounds like VAD is enabled or ECAN interferes.

But in the case with Modems calling, the sshhhhhhh sound is heard just for a half second and then completely gone.


Ahhh I C....  If you configured the PRI using CCA why not do the same for your FXS\FXO ports?  CCA is way easier!!

VIA CLI it should be done line this.  If you dont create a dial-peer to turn off VAD you might run into trouble.

Global config - The gateway will detect the T.30 protocol being used and convert  it

Gateway(config)#Voice service voip

Gateway(conf-voi-serv)#fax protocol pass-through g711ulaw

No need for VAD because G.711 does not support it.

Gateway(config)#voice-port 0/1

Gateway(config-voiceport)#no echo-cancel enable


Gateway(config)#Voice service voip

Gateway(conf-voi-serv)#fax protocol t38

Gateway(config-dial-peer)#no vad (on the dial-peer that the fax will be using)

Gateway(config)#voice-port 0/1

Gateway(config-voiceport)#no echo-cancel enable

Manual config on a dial peer by dial peer basis

Gateway(config)#dial-peer 20 voip

Gateway(config-dial-peer)#fax protocol t38 or system

Gateway(config-dial-peer)#no vad

Gateway(config)#voice-port 0/1

Gateway(config-voiceport)#no echo-cancel enable


if you use CCA you can just configure the FXS port as a Fax line. That can be done via TSW (Telephony Setup Wizard) or under the users tab.  CCA + unsupported CLI commands = disaster

Fax Protocols

Cisco - Proprietary protocol can only be used between cisco gateways. Not used anymore

T.38 - Industry standard.  FAX-----T.30-------Gateway--------T.38-------Gateway-------T.30--------FAX

Pass Through - Gateway uses G.711 (64Kbits) to transmit the fax.


Ok.  so my setup is FXS ----> PRI.  no voip involved here, so no t.38 etc.

But I did take your advice to use CCA which resolved my problem but opened up another question.

In CCA(v2.2.4) it asked me to go to "ports->analogue port settings" to configure as "FAX" or "Common area phone".  The port was currently set to "user phone"

Although I found no difference from "fax" or "common area phone" I did find that "user phone" uses "stcapp/ephone-dn/ephone" and the other two do not. but instead use a "destination-pattern" right in the dial-peer.

So Common Area and fax (non stcapp) work, where as "user" (stcapp) does not and audio is cut out.

So now my question is:

What is 'stcapp' ?  what / when should it be used?  and WHY does it cutt out the audio?

aside: using stcapp uses up a license on the UC. where as non-stcapp doesn't. is that statement true?


STCAPP stands for SCCP Telephone Control Application.  Its there to enable the FXS phone to be able to dial * to access features an IP Phone has. Some of those feature would be Call Pickup, Cal Park and DND. It tells the UC500 to register that port and it creates a dial peer for it.  That's why it uses a license.

So this doesn't seem like normal behaviour?  Or is it written somewhere that when using Modems, STCAPP should not be used?


STCAPP is to enable a phone connected to a FXS line to be able to access features using the * key.  Modems and faxes don't use that function.

STCAPP seems to interfere with modem audio modulation for some reason. Make sure you set it up as either "common-Area Phone" or "Fax". You'll end up with a destination-pattern in a dial-peer for pots. This will also remove the ephone thus the need for a license.


Did you manage to solve your problem removing stcapp from FXS configuration?

Because i have  one 2921 router with one EVM-HD-8FXS/DID   configured with stcapp  and i'm having some problems with modens sometimes didn't work. receiveng calls from PSTN.

I'm using CME 8.1.

Also in  my configuration shoul i have this?:

voice service voip
  fax protocol t38 version 0 ls-redundancy 0 hs-redundancy 0 fallback pass-trough g711alaw.
modem passthrough nse codec g711alaw


Best Regards


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