I have hung up phone,but the channel is still active ,why?

I have hung up the phone,but the chanel still active .So I cant call the user again.Why? Is it because of low network speed?

sterisk1*CLI> sip show channels
Peer User/ANR Call ID Seq (Tx/Rx) Form Hold Last Message 1002 0e01677f749 00102/00000 ulaw No Tx: ACK 1001 4F5114E3-31 00101/07227 ulaw No Rx: INVITE
2 active SIP channels

Anybody can help me ? Thanks !
Best Regards

You need to post more details, like asterisk version, what kind of phones you have.