20 seconds til voice transmission starts

Hello everybody,

I’m experiencing a strange problem with my Asterisk. I hope you can help:

Asterisk is running at my company behind NAT. Ports 5060 and 10000-20000 are being forwarded to it. I have put the router’s external IP-address into externip in sip.conf. At home I’m using an AVM FritzBox Fon WLAN 7050 which is registered with the Asterisk at my company.

When I try to call Asterisk (or a phone connected to the attached legacy-pbx) from home, it’s ringing normally and I can hear my opposite. But it takes about 20 seconds until my opposite hears me! When I call the same number again staight after, everything is working fine from the beginning. Also, calls from the company to my home are working perfectly.

I’m greateful for any tips!

Regards, Lius