[Asterisk-Users] Hanging SIP Channels

[asterisk fpf]

Hey folks,

i have a problem with hanging SIP channels which are used for agents. 
The channels remain opened in
a random behavior:

show agents:
43           (The Agent) available at 'hd1 at SIPAGENT' (musiconhold is 
'default')

show channels verbose:
Channel                  Context              Extension        Prio 
State   Application  Data                      CallerID        Duration 
Accountcode BridgedTo
SIP/hd1-4c1c         SIPAGENT             s                   1 Up      
(None)       (None)                    
hd1                                  (None)

sip show channels:
Peer             User/ANR    Call ID      Seq (Tx/Rx)  Form  Hold     
Last Message
192.168.97.19    hd1         03af1d1458a  00102/00001  alaw  No       
Rx: BYE

This channels stay's up while no application is reading/writing from/to 
it. So "soft hangup" the channel is NOT possible.

Is there any solution on that problem ??

Greetings,
    Michael