the first call (even *600) after registration is not working?

[casch]

Hi,

I have a stange problem - maybe s.o. can help?

The first call after registration of the phone at voxalot fails allways.
I hear everything but nothing I say is 'broadcasted'.
Even if I call *600, the echo test: I hear everything, but nothing I say is repeated. I hang up and redial *600 fine, no problem!

Same thing if I want to make an outbound call after registration - nobody hears me.
If I call the second time - now it works?

NAT is set to NO. voxalot's stun is entered.

My WLAN-phone behind a WLAN-Router (dd-wrt).
The phone's MAC is set to the max pass-through (QoS: Premium).
The phone registers first at the router, hereafter at eu.voxalot.com: No problem.

What can I do that even my first call after my phone's registration at eu.voxalot.com works correctly?

Thanks in advance
Carl