I use custom SIP outbound proxy which just logs and analyses SIP requests and responses, forwarding them from Zoiper to real proxy and the other way.
On Linux and Mac everything works fine: Zoiper is able to register, send and receive calls, and my proxy does what it should.
On Windows Zoiper is able to register, send and receive calls too, BUT there is no sound at all: I can't hear the client, and the client can't hear me, though SIP requests and responses are sent and received pretty fine. And if I stop using my proxy - the sound gets back again.
I use Zoiper 3.
What could I change in Zoiper's or Windows' settings to get the sound back and still use my proxy?