No voice on internal calls

We have FreePBX (Current Asterisk Version: 16.6.2).
Extension numbers are configured as SIP.
When trying to make an internal call, there is no voice. The call ends after 31 seconds and we see in the logs
chan_sip.c: 29861 check_rtp_timeout: Disconnecting call for lack of RTP activity in 31 seconds

Phones sets are located in a separate VLAN (, the FreePBX server is located in another VLAN ( via ipsec connection.
We tried to enable/disable NAT. The result does not changes.

The identical configuration in the network topology without ipsec works fine.

Where to look for the answer? Thanks!

Probably need more information, but my guess is that the VLAN subnet is not listed under Local Networks in Settings, Asterisk SIP Settings.

Thanks a lot! It looks like your advice helped solve the problem.
Added a second subnet to the Local Networks settings.

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