The first 2 seconds of pickup a call are lost

We use asterisk 1.4.17 on OpenSuse 11 with an ISDN-Card AVM B1. The calls in our company are going on with SIP-VoIP. Outside we use ISDN. Our problem is that when we make a call in or outside, the first 1-2 seconds are lost. Because of this we or the other partner can`t hear that we are on the phone. The rest of the call is going very well without any problems. All our Switches or Firewall have QoS for VoIP installed. The Phones that we are using are from SNOM. (320 and M3).
Does anybody have a solution for that problem?