3CX sends DTMF between Extensions&Trunk but between Trunk/Trunk (Intercom)

[Roeller]

Hello,

In my job to make a Migration Tutorial for people migrating from Asterix/FreePBX to 3CX, I ran into the following problem:

• 3CX can forward DTMF tones from a SIP Trunk to an Extension (IP Phone).
• 3CX can also forward DTMF tones from Extensions to the SIP Trunk Provider.
  
• 3CX can not forward DTMF tones received on a SIP trunk to another SIP tunk.

Example: When you have an Intercom or another device which registers as a SIP TRUNK to 3CX. Then 3CX can NOT forward DTMF tones incomming from the SIP Trunk Provider to the Intercom (which is a SIP Trunk and not an extension).

I tested the above thoroughlythe past days and made Wireshark traces. In the trace you can see DTMF tones (as RFC2833 payload) coming in from the SIP Provider to 3CX, now 3CX does forward them to Extensions but NOT to other SIP Trunks (aka, the Intercoms). For intercom systems it is very important to have DTMF working because of opening a Gate or Door is one of it functions.

See the two drawings below for the concept. I am wondering whether this is a 'bug' or 'by design'.
(Click to enlarge!)

When the Intercom Rings a normal extension, the extension can send DTMF and 3CX forwards this DTMF to the Intercom.



When the Intercom Rings a mobile phone (via the SIP Trunk Provider), the Mobile phone sends DTMF which is received by 3CX but 3CX does not forward this to the Intercom. In my Wireshark traces I see in the SIP/SDP block that all devices support RFC2833 (telephone-event-101).









(sorry for the attachments below, it is neccesary to display the above image and I can not hide them)

 

[lneblett]

A little confused -

You have a SIP endpoint that is an intercom, which to me, I take to be like an extension. You specifically state that it is a trunk and I am wondering why? Can you provide the make and model of the intercom? The diagram shows all communication going thru the SIP Trunk provider and then presumably thru 3CX (given the LAN indication), which makes it unclear to me the reason for the device being set as a trunk. How does your cell phone, as shown, reach the intercom?

As a note of caution, there is no need to publish the IP addresses as it may invite others to "play/test/hack". If you do need to publish an IP use a pseudo number that doe snot reflect the true number, but lets us know if private or public.

 

[donbru01]

Hi , i have one years old same issue , solved managing trunk with delivery audio from pbx

 

[Roeller]

Thanks for the replies!

The Intercom is a GIRA DCS-IP Gateway (TKS IP Gateway).
For example, here are the settings with a FritzBox: installing-fritzbox-Gira (in German) and here are the settings for XXTer (English).
The manual of the Intercom itself is here (English)

Mobotix and Commend Intercoms also use SIP Trunks instead of an Extension.
So the 3CX Server registers to the Intercom instead of the Intercom registering at the 3CX Server.

When someone presses a button on the Intercom, there is an 'incomming call' (incomming route) on the 3CX.
This incomming call is forwarded to Extension1021, which has the option 'Ring my extension and my Mobile at the same time'.
Now I can pick up the Mobile phone and press the code to open the gate.. however,.. DTMF arrives at the 3CX server but it does not forward the DTMF to the Intercom.

In my drawing I used 'random' public IP addresses

 

[donbru01]

Sorry i haven't see this post .
Youhave solved this issue ?

 

[NickD_3CX]

...which has the option 'Ring my extension and my Mobile at the same time'. Now I can pick up the Mobile phone and press the code to open the gate...

When an External Numbers rings as a result of the 'Ring my extension and my Mobile at the same time' option, the External Number cannot send DTMF tones. To be precise it can, but they are intercepted by the PBX because it has a built-in Mobile Agent Service.
More about this in the following, section "Mobile Agent Service" (use Ctrl + F)
http://www.3cx.com/blog/extension-user-manual/dial-codes/

If you use any other method to call the External Number apart from the 'Ring my extension and my Mobile at the same time' option, like using Forwarding Rules or, seeing that the Intercom is registered as a Trunk, an Inbound Rule to send it directly to another number, then this should work.

 

[Roeller]

Thank you NickD_3CX !! That was exactly the issue. 3CX did catch the DTMF and did not put it through. This was because of the Mobile Agent you are saying.


So
- Using 'Ring my mobile at the same time' was not an option.
- Using a Ring Group with an extension in it (which has a forwarding rule to my mobile) also didn't work. Because Ring groups don't work with Extensions Forwarding rules.

Solution:

I did set up a Call Queue with the extensions in the house AND some 'external agents' which are my mobiles.
Now I can always pick up my IP-phone when I'm at home or my mobile when I'm on the road.. and use DTMF to open the gate/door remotely. I've even set the Call Queue to a 'Digital Receptionist' after about 50secs, which says: "Sorry,. I could not open the door at the moment or I'm not at home" (just in case I can not pick up the phone).

Thanks again!