Vpn internal calls one way audio

We have 3 sites connected by vpn.
In site A there is the FreePBX and 2 sip phones.
in site B and C there is 1 sip phone.
There is no problem with inbound and outbound calls.
But with internal calls there is a problem :
Site A can call site B and C.
Site B can call site A and C.
If site C call site A or site B, there is no audio from site C, only from other side.