OpenVpn with yealink T26 phone

[charneval]

Hi to the whole forum.
I'm trying to run a yealink t23 phone connected in vpn via openvpn with a pfsense that protects a freepbx switchboard.
The vpn is established correctly but the voice coming out of the yealink phone is not clean and is disturbed.
While you hear the incoming call perfectly.

The codecs I can test in the switchboard are the g711a and g711u.

What do you think I can check in the openvpn configuration?

Thank you

Andrew

This is my configuration :

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