asterisk users - Aug 2003 - X100P disconnecting without any reason

Hi,

I have one X100P card connected to the PSTN line and two analog phones
connected to an ATA v2.16, SIP mode).
When a call is received from PSTN, after about 5-10' the call is closed
without apparently any reason. I have increased the value busycount to 10
and the problem persist.
A pure IP call (local or even remote) is never closed by himself.

There is something else to check?

Thanks,
Dan