First answer is silent then rings back - Please Help

I’m having a strange issue happening on two different installations.

The phone rings and the Caller ID is correct.
The user picks up the phone and no one is there (slient), so they hang up.
Once they hang up the phone rings again but this time the Caller ID says “Unknow” or “0”.
The user picks up and the caller is there.
The caller says they did not call back but did hear a brief pause during the ringing.

Note: This happens on multiple phones/extenstions.

OS: CentOS release 5.3 (Final)
Asterisk 1.4.22-4
Trixbox v2.6.2.3
Grandstream 4108 FXO Gateway
Grandstream 2020 and 1200 IP Phones

System Specs:
Intel® Xeon™ CPU 2.40GHz
1GB Memory
20GB Hard Drive
10/100/1000 NIC

Any help I could get on this would be great because I’m at a loss.