[Asterisk-Users] Strange Problem

[Asterisk Learner]

I am experiencing a strange problem and wanted to know if someone has faced any similar issues or could provide me with a way to counter this problem. I am in the process of experimenting with asterisk and trying to setup a basic functional system. I have one TDM400P (single port) and one X100P. I am using one analog phone connected to the TDM400P and I also have a couple of Xlite SIP phones configured. I can make calls out to the PSTN and I can also receive calls.   The problem happens when someone from the outside (PSTN) calls the Asterisk box. I have asterisk configured to forward the call to Zap/2 (analog phone). Zap/2 rings and I can talk to the person on the other end but if I hang up first then the other end does not see as the call being hung up. Asterisk CLI shows that Zap/1-1 (FXO) hungup but for some reason the other end thinks that the call is still up and does not disconnect unless the person hangs up himself. The confusing part is that if I initiate the call then this problem does not happen.   Can someone tell me what is happening and how to resolve this issue?   Thanks