Understanding PBX delivers Audio?

[FBHLMike]

We are new to 3CX and have just implemented it across our company. Roughly 300 devices in roughly 40 locations. We are primarily using Yealink T46S devices.

During our deployment we encountered an issue where extension to extension calls had no audio. The "Fix" was to turn on PBX Delivers audio.

In our troubleshooting we have done Any Any firewall rules internally with no results. We have disabled AV with no success. Ideally we do not want to move forward with this configuration.

A little more information about our setup. We are using a cloud hosted 3CX system currently, but that will be changing soon. We are not using any form of SIP ALG and other than this issue, all has worked flawlessly.

Where should we be focusing our attention to track this issue down?

 

[Saqqara]

Need more info,

You have cloud hosted 3cx, how are the remote phones connected, how many at each site

If you have site to site vpn, you have configured phone as local.

stun config - does each phone have different rtp ports (min of 12 rtp ports), setup the correct portforwarding on the firewall to forward the rtp ports to the phone ip address

Phone 1 will have 14000 to 14012 rtp port numbers
Phone 2 will have 14013 to 14025 rtp port numbers
Phone 3 will have 14026 to 14038 rtp port numbers,

sbc config - do your remote sites have a SBC server. maybe worth looking at https://www.3cx.com/docs/3cx-tunnel-session-border-controller/

setup info for phone https://www.3cx.com/sip-phones/yealink-t4-series/

 

[FBHLMike]

Hello,

There is no VPN between us and our 3CX cloud host. We will be bringing this system in house to our local data center in the future however.

We are setup for SIP/STUN on each of the phones.

I am not sure on the assigned RTP ports. I know this much, we do not have predefined port forwarding rules on our firewall where we are forwarding RTP ports to specific phones.

Also, to my knowledge, we do not have an SBC.

 

[leejor]

Ideally, at a remote location, any set to set calls (within that location), will go directly between the sets once the call is established. Not all routers are capable of keeping track of the audio ports to be used. The result is one way, or no audio on the calls. Using PBX delivers audio is really a last resort "fix", that should be avoided if possible. Using an SBC, or VPN, as suggested, saves a lot of bandwidth, as the audio does not have to go back though the PBX. It also allows other 3CX features to function as they should.

 

[cobaltit]

So PBX Delivers Audio is exactly as it sounds. The PBX stays in the audio path the whole time. If you have no audio with extension to extension calls, that means you have network configuration that is preventing the two endpoints from talking to each other. This can be for any number of reasons, none of which are 3CX specific. The link below has diagrams showing various network configurations and how the SIP and RTP traffic flow in those scenarios.

https://www.3cx.com/blog/docs/network-configurations-supported-3cx-phone-system/

 

[Saqqara]

Info on stun setup - https://www.3cx.com/3cxacademy/videos/intermediate/configuring-remote-extensions/

Info on sbc setup - https://www.3cx.com/3cxacademy/videos/intermediate/configuring-remote-extensions/

If you have no site to site vpn's I would look at installing SBC at each remote site

 

[YiannisH_3CX]

Hello @FBHLMike

As you will see in the link provided above by @Saqqara when you are provisioning phones using STUN the PBX delivers audio has to be enabled in order to have audio from extension to extension. The reason is that the PBX only sees the public IP of the phones as they register and when 2 phones are behind the same public IP the PBX will see that it is the same and will tell the phones to talk directly to each other. However the PBX does not know the local IP of the phones so the phones will try to communicate through the public IP and most of the firewalls do not allow loopback traffic so no audio will pass to the phones.
The PBX delivers audio option will keep the audio going through the PBX so you will not face this issue. Provisioning the phones through an SBC also solves this issue as the SBC acts as the proxy to each site and the audio is handled internally when 2 extensions are behind the same SBC.

 

[FBHLMike]

Thank you for your help on this issue! I think you all have given me all the information I need to correct this issue.

 

[YiannisH_3CX]

Glad we could help