Sometimes caller can't hear me

Hi all.

I’m running Freepbx 14.0.16.4 on Raspberry PI 3b+
Asterisk version is 13.29.2

I use an OVH SIP account on port 5060.
Locally I use an AASTRA 57i device.

Sporadically, on incoming calls, I got an issue where I can hear the caller, but the caller can’t hear me.
When this occur, the caller calls again and it works.

Any idea ?

Regards,
Laurent.