Conference drops after 30sec

We use S705 devices connected to our FreePBX using VPN.
Our only problem is for internal conference (for the moment we never use it with external calls).
Only the first 2 participant can stay on the conference, the third is disconnected after 30 sec.
Same problem with Zulu connected directly without VPN.
I’ve tried to endable debug, but I can’t understand where the problem comes from.
Any other calls works fine.
Thanks alot for your help !

Provide a log of the issue.

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