Showing results for 
Search instead for 
Did you mean: 

Forward from ISDN to VoIP


How do I forward incoming calls from ISDN to VoIP?

We have a few numbers which are terminated on our ISDN 2 and 30 trunks and I'd like to forward them to a VoIP server.

We're running Cisco IOS Software, C3900 Software (C3900-UNIVERSALK9-M), Version 15.0(1)M4, RELEASE SOFTWARE (fc1) with CallManager Express.

If someone could take a look at the following dial-peer config and point out the issues, that would be great.

So, basically, all calls arriving onISDN for 123456789 should be forwarded to 123456789 but then over VoIP.

dial-peer voice 2001 pots
description From ISDN
incoming called-number 123456789
port 0/3/0:15

dial-peer voice 2002 voip
description To VoIP Server
destination-pattern 123456789
session protocol sipv2
session target ipv4:
session transport udp
voice-class sip dtmf-relay force rtp-nte
voice-class sip profiles 1
dtmf-relay rtp-nte
codec g711alaw
no vad

4 Replies 4

VIP Alumni
VIP Alumni

Hi Grant,

The 1st thing you need to do is to find out what signalling digits are actually being received from the CO on your ISDN link. I doubt very much that you will be receiving 9 digits, usually more like 6 or 4.

Try a q931 debug and make a call externally to the target number

Hopefully you should see an incoming call setup which will display the digits you are receiving

term mon
debug isdn q931

When finished no debug isdn q931

Once you know the digits you can then transpose to what ever destination number you need via
translation rules etc on your router or in you call server at the other end of the voip.


Regards, Alex. Please rate useful posts.

Hi acampbell,

The digits received are less than 9, this was just an example to hide our public PSTN numbers. The called number (call) needs to be forwarded 1on1 to the VoIP server so I don't need any translation rules. I'm just looking for verification if this config is valid and would work, before I apply it. Any faulty changes applied would render our company unreachable, which is a no no :)

Ayodeji Okanlawon
VIP Alumni
VIP Alumni

What is the destination device? is that a phone on the ccme? Do you have any issues at the moment with this configuration?

Please rate all useful posts

The destination device is a VoIP server (AudioCodes SBC) which is connected to our Skype For Business servers. 

I haven't tested this configuration yet, since any changes applied are immediately applied. So that's why I'd like to verify this config before I apply it, because an error would result in our company not being reachable by phone.

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: