[HELP]Having problems with callee hangup

Hi all,
When i make a call from SIP Phone which registered to Asterisk to PSTN Phone, then the PSTN Phone answered, it works fine.But when the PSTN Phone(callee) hangup, caller(SIP Phone) also connected. It looks like:
| SIP caller| -> | asterisk | -> | NAT | -> | VOIP provider | -> | PSTN callee |