asterisk users - Jun 2008 - FXS: two rings, then it answer and hangup

Hi All;

I do not know why this happen, if anyone faced it and knows any idea about it:

I have analoge phone connected to FXS port, when we dial its extension, the
phone ring first ring, second ring, then it automatically answer the channel and
hangup (as I see in the CLI), anyone can advise the reason for this?

Regards
Bilal