[Asterisk-Users] the pstn line is noisy, busydetect can detect hangup?

[dev2003 at mail.ustc.edu.cn]

the pstn line is noisy,busydetect can detect hangup?
I have a X101P connected to my analog PBX. I can′t
detect hangups on this line
While the line is connected to the PBX, I can never detect busy and the
line hangs at the end of every call.

busydetect Alg.  compares silence with tone .the pstn line is noisy.
can it work?
how improve busydetect alg?
how filter noise in dsp.c?