Solved External to Internal One way audio drop outs for Internal User Only

[Glenry]

Issue: One way audio drop outs

Scenario: Call is established from external pstn line via Patton SN4114 to internal Yealink T4XS phone on a remote site using SBC. Drop outs are reported by internal staff member. External party is crystal clear. Audio drop outs are around a second in length and can vary in frequency from every second to every 10 seconds. We tested the same workflow with the Yealink T4XG and Yealink W52P phones and did not have this issue for an office of around 50 endpoints. If we reprovision the S series phone to not use the SBC and work as remote IP phone via direct SIP the issues are also not present.


Data Centre

3cx version 15.5.3849.1 All Patches up to date


Remote Site (No NAT Required as Layer 3 WAN in place)

SBC; 3CXSBC15.msi; revision {559651FC-E8AA-47B9-8679-3EBF3686B5CB}

Gateway; Patton SN4114/JO/EUI; Hardware Version 4.4; Software Version (R6.T 2017-05-08 H323 SIP FXS FXO)


Phones

Yealink W52P 25.80.0.15

Yealink T42G 29.81.0.110

Yealink T42S 66.81.0.110

Yealink T48G 35.81.0.110

Yealink T48S 66.81.0.110

 

[eddv123]

Hi,

Firstly your setup is not a 3CX "Supported" method however I have got both Patton and beroNet boxes working across the SBC without issue before. It is normally the simple case of pointing the SIP Registrar at the SBC instead of directly at 3CX.

That being said, the first question has to be why have you gone to all this effort instead of employing SIP Trunks with your cloud PBX it would be much simpler ?

Can you answer the following:

* Is the gateway setup manually
* If no have you auto-provisioned and then altered the settings
* What firmware is the Patton running
* What version of the SBC do you have, Windows, Linux or R-Pi

There are debug commands you can run from gateway however, it would be of interest if you have managed to get a PCAP trace of the issue as you can playback and see the audio path.

 

[Glenry]

Hi @eddv123,

Thanks for your response.

In answer to your first question it is a staged migration from initial PSTN and ISDN lines at the remote sites into SIP trunks in the data centre. Why stage it? Because of time constraints and lead times with the SIP trunks forcing our hand. Why not use supported cloud PBX? Due to contractual pricing with another SIP trunk provider.

Can you please explain how this is unsupported? The Patton gateway is communicating directly to the 3cx without any NAT due to layer 3 MPLS being in place which as far as the gateway cares (presuming WAN connection does not have any latency or jitter) would be no different to having it on a different vlan to the pbx in the data centre? Unless I am missing something. The remote phones proxy through the SBC.

Gateway was auto provisioned and then adjusted to add in a few settings the auto provisioning does not do, such as routes for access to management interface from other vlans and a changes to the Caller-id type.

Patton gateway is running R6.9 2016-12-27 H323 SIP FXS FXO

SBC is the windows version running on Windows Server 2012 R2.

 

[eddv123]

Hi Glenry,

OK that makes perfect sense why you are doing it this way.

Just to confirm what I mean when I say "supported". See the below statement from this guide:
https://www.3cx.com/voip-gateways/patton-smartnode-sn4112/

"The device must be local to 3CX, in the same subnet and/or locally routed subnet (including site to site VPN)"

Simply meaning you would not get any support from 3CX if you were to contact them about using a gateway across SBC or using Direct SIP/STUN. I however know this should work OK as I have set it up myself with FXS and ISDN gateways BRI/PRI.

The other option if you have an MPLS network in place - as this is similar to having a VPN anyway (private network in the cloud) is to do away with the SBC/STUN methods and route according to your network. This would be preferable.

Normally the configuration files generated by the Patton assume that the gateway is local, so it doesn't create a default gateway.
This wouldn't be needed if you were using the 3CX SBC (SIP Registrar change only) however if you wanted to route across your MPLS or a VPN it would require one.

In the Patton in the Networking / IP / Routes page please add an entry like this.

0.0.0.0 0.0.0.0 <local router IP> metric 2

In addition to this, you'll need to permit direct access for the local IP of your patton gateway to port 5060 including RTP (this will need to specified on your FW in front of 3CX).

 

[Glenry]

@eddv123 You were correct in that having the Patton SN4114 on a different vlan did not support the out of the box settings for this gateway in the 3cx portal.

Remote support session with packet capture data highlighted the cause was due to the Patton SN4114 FXO being on a separate vlan. The following settings on within the 3cx SIP trunk resolved this on all sites.

ON ... PBX Delivers Audio
OFF ... Supports Re-Invite
OFF ... Support Replaces​

 

[YiannisH_3CX]

Glad the issue is resolved and thank you for updating the post with your solution.