Call hangup immediately

I am using CentOS 4.5 and Asterisk v1.4.18.
I observer the asterisk CLI and sometime i found notices :

...
[Nov 18 09:31:53] NOTICE[13247]: chan_unicall.c:2516 handle_uc_event: Unicall/108 event Detected
[Nov 18 09:31:54] NOTICE[13247]: chan_unicall.c:2516 handle_uc_event: Unicall/80 event Detected
[Nov 18 09:31:55] NOTICE[13247]: chan_unicall.c:2516 handle_uc_event: Unicall/108 event Far end disconnected
[Nov 18 09:31:55] NOTICE[13247]: chan_unicall.c:2788 handle_uc_event: CRN 32788 - far disconnected cause=Normal, unspecified cause [31]
[Nov 18 09:31:55] NOTICE[13247]: chan_unicall.c:2516 handle_uc_event: Unicall/108 event Drop call
[Nov 18 09:31:55] NOTICE[13247]: chan_unicall.c:2516 handle_uc_event: Unicall/108 event Release call -- Unicall/108 released
[Nov 18 09:31:58] NOTICE[13247]: chan_unicall.c:2516 handle_uc_event: Unicall/80 event Offered
[Nov 18 09:31:58] NOTICE[13247]: chan_unicall.c:2640 handle_uc_event: CRN 32786 - Offered on channel 0 (ANI: 222477047, DNIS: 37302100, Cat: 0)
[Nov 18 09:31:58] NOTICE[13247]: chan_unicall.c:2516 handle_uc_event: Unicall/80 event Accepted
...

Do you know why the call at 108 channel drop immediately ?