I think you are using the one-stage dialing method to dial to the destination, right?
If you dial to PSTN via FXO port with one-stage dialing, how does the FXO know the PSTN side had picked up the phone?
The FXO will know the remote pstn side had picked up the phone by "ring back tone detection"
When you dial the PSTN by one-stage dialing, the 3802 will check the routing and send the call to PSTN directly, then the PSTN side will start to ring and you can hear the ring back tone.
Actually, the media channel has opened and the ring back tone was coming from PSTN side.
Then the 3802 should have the ability to detect the ring back tone.
If the PSTN side pick up the phone, the CO will stop the ring back tone, and the 3802 will know the remote PSTN side had picked up the phone because the ring back tone is stoped, and 3802 will send the 200OK message to caller.
You can check the ring back tone table by "tone -print" command.
There are four records in the ring back tone table.
The 3802 can detect the ring back tone status if the tone frequency can match one of these four records.
In different location or different PBX, the ring back tone frequency should be different.
So the 3802 has the "auto connect" function to avoid the above issue.
The default auto connect time is 5 seconds and you can disable this function by "sysconf -connect 0" command.
I think you can disable the "auto connect" function and make sure the ring back tone frequency is already setted in the ring back tone table correctly; then test again.