Why does not remote PSTN Gateway work with SBC?

[Brotto Jr]

Hi There,
I have 3cx PRO edition 15.5 running at Google Cloud.
My remote office has fixed IP and it is running SBC for 8 devices (IP Phones) - SBC on RPi 3 Model B Plus with last version of SBC. All of devices are working properly.
I am trying to add a PSTN gateway... but I heard the PSTN gateways do not work with SBC on the same public IP. Why does not it work? Workarounds?
Tks.

 

[leejor]

One big difference with a gateway as opposed to an ATA (or SIP phone), is that, along with registration, in the 3CX trunk settings, you specify where (IP and port) to send a call (outgoing towards the gateway). Phones simply have to register, which contains all of the information that 3CX requires to "contact" them. An SBC is the middleman, which makes things easier for a SIP phone, especially when communicating set to set, at the remote location, but it complicates things if you are trying to use a gateway, which normally communicates direct to the PBX.

I have used a single line gateway, with success (no SBC), and using VPN should work.

In theory, if you can get your remote router to correctly handle (route), not only the communications ports (5060, 5061, etc.) but the voice ports too, it should work.

 

[Brotto Jr]

@leejor,

I am happy to hear that!

Here follows my scenario and one more question related to:

My scenario:

Phone System: Google Cloud with standart instalation, version 15.5
Public IP (456.456.456.456) - example

Remote Office

a) Router:
Public IP 123.123.123.123 (example)
IP: 192.168.1.1

b) SBC over RPi model 3BPlus
IP: 192.168.1.42 (static IP), running pretty well SBC thru port 5090 with Phone System Google Cloud, pointing to 456.456.456.456, for 8 IP Phones.

c) Gateway PSTN - Device 1: using FXO and setup ok on 3CX as Trunk 1 Gateway Port 5062
Device IP: 192.168.1.43 (static)
Device Sip Port: 5062
Device RTP port: 9452

d) Gateway PSTN - Device 2: using FXO and setup ok on 3CX as other Trunk 2 Gateway Port 5064
Device IP: 192.168.1.44 (static)
Device Sip Port: 5064
Device RTP port: 9454

I will port forward 5062 to 192.168.1.43 and port foward 5064 to 192.168.1.44 in order to route to geteway devices.

So, I missing port forwarding 'voice/RTP port' for each device (port 9452 and 9454) on my remote office Router.

Once there is no way to setup RTP port on 'Sip trunk > gateway > devices' on 3CX, my questions:

1) How will 3cx system know that it must use port 9452 for voice/RTP for Trunk 1 and 9454 for voice/RTP for trunk2?

2) Does 3cx phone system 'reserve' ports 9452 and 9454 in order to not use for other Devices?

Tks!

 

[leejor]

1) How will 3cx system know that it must use port 9452 for voice/RTP for Trunk 1 and 9454 for voice/RTP for trunk2?

2) Does 3cx phone system 'reserve' ports 9452 and 9454 in order to not use for other Devices?

That will be the issue, and can cause audio problems when using more than one STUN device (a phone too) behind a remote router. That is why most users opt to go for an SBC. While the "control" ports can be forwarded, you are relying on the router to sort out the audio ports as they are negotiated. You are going to have to do some trial and error testing, to see if it all works, especially two calls at a time, and originating from the PBX end, that will be the real test. Others may have some suggestions, but the big variables are going to be the make/model gateway, and the remote router settings/features.

 

[eddv123]

+1 despite being un-supported I have had multiple type and brand of PSTN gateway (FXO/S and ISDN BRI/PRI) working via a hosted PBX.

I cannot see what brand you are trying to use however ?

 

[Brotto Jr]

HI @eddv123
My remote router is a Draytek Vigor 2926.
And both remote PSTN gateways are Linksys SPA3102.
So far just PSTN-to-VOIP are working fine.
Any help would be very appreciate.

 

[leejor]

With just one gateway at that location, you should be able to forward all possible 9000 series ports, that may be used, to the IP of the gateway, as all of the other sets run through the SBC. Using two is probably going to be the problem, for calls from 3CX to the gateway. If you had a multiline gateway, then you would only have to deal with one IP, for all PSTN lines. I suspect that is what eddv123 used.
As I said, I have had it working for just one gateway, and it was a 3102.

Not sure it would work, but...turn on the 3102 router function and have the second 3102 pick up an IP from the first? Send all 9000 ports to the first? Just a thought. Have not tried it and may just be wish-full thinking

 

[eddv123]

Hi Brotto Jr,

I concur with Leejor, a multi-line gateway was used not 2 x gateways.

I have not tested 2 gateways across a single SBC, however if this is causing you an issue then you could try multiple SBC's (this is supported) and split the subnets. Please note this is only theoretically speaking and I have not tried this myself but would be interested to know if it does work.

Obviously in the gateways you would point them at each specific SBC IP address instead of the 3CX IP address.

 

[Carlos Symonds]

You can call the Linksys support team anytime at 800-531-1267. They will help you to fix the every issue of the Linksys Router.

 

[leejor]

You can call the Linksys support team anytime at 800-531-1267. They will help you to fix the every issue of the Linksys Router.

It doesn't really appear to be a router issue....

Using an SBC, for just the gateways sounds intriguing. The issue I see, is calls from 3CX routing to the gateway, as the IP and port is put in the 3CX trunk settings. While gateways do register, they don't function quite the same way as an ATA or SIP set. Worth a try if you have a spare SBC sitting around