One-way audio, but it's interoffice

First, let me establish the system details:
FreePBX on bare metal – Up to date modules
All extensions are PJSIP

All IP Phones are Grandstream GXP2160’s with varying versions of the firmware.
There are seven IP phones in this system, all configured with FreePBX’s EPM - nothing custom about the configuration.
The firmware on the phones does vary. The firmware version on the problematic phones is version Version, which I believe is the latest.
The preferred codec is G729

The network is a very vanilla flat network with nothing remarkable in place.
The network equipment is a Unifi setup. One USG Pro 4 and a US-24-POE

Now, let me explain the behavior I see. All my inter-office calls work just fine between all the phones except for two of them, which have a one-way audio issue. In the case of these two phones, they cannot hear inbound inter-office audio. The outbound inter-office audio from these two phones is working.

I did turn on verbose PJSIP logging in Asterisk. I also performed a PCAP and reviewed it.
I didn’t see anything remarkable in those logs just yet, though I will be honest I may not know what to look for.

I find this situation rather odd as I’ve got many identical deployments and have never experienced this issue.

I can post whatever logs or PCAPs would be helpful on request. Any input or advice would be much appreciated.

I would inspect the SDP headers of the Invite and 200 ok messages to see if the media addresses audio should be sent to are actually the ones you would be expecting, i.e. the addresses of the phones.

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This was good advice, thanks.
I see that my problematic phone tried to negotiate a media bridge on port 4, which is obviously not inside our RTC range. Now the question remains, why would this happen? I’m looking into it now.

Changing to an older Grandstream firmware did actually fix this, for anyone who finds this post in the future.

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