Need advice: USW Pro dropping VoIP calls

I replaced two aging Cisco switches for two USW Pro 48 PoE switches. All the workstations operate on the default VLAN and I configured a VLAN 10 for the Yealink VoIP phones. VoIP traffic is routed over IPsec to our FreePBX box located at another site. All other configs on the USW are at their defaults. The firewalls on both ends are pfSense+.

Inbound and outbound calls work fine, but if you place a call on hold, both ends go mute. I narrowed the issue down to the USW’s by plugging a VLAN 10 tagged phone directly into the gateway, and the issue was resolved. I am thinking it is a NAT issue.

Here is a simple topology of the setup. Any ideas?

                                         IPsec over
             SITE A                      Internet           SITE B
                                     │               │
┌──────┐        ┌───────┐       ┌────┴───┐       ┌───┴────┐       ┌─────────┐
│Phone ├───────►│USW Pro├──────►│Netgate │       │Netgate ├──────►│FreePBX  │
└──────┘        └───────┘       └────────┘       └────────┘       └─────────┘
VLAN 10         VLAN 1
                Trunked VLAN 10

Welcome to the forums. We hardly use any USG devices so not an issue I have encountered, hope someone has a suggestions.

Usually when I’ve seen (or heard, I guess) problems with disappearing audio it has to do with RTP packets not getting through to the server. You might be onto something with your NAT theory. I’m surprised it’s just when the call is on hold, though.