asterisk users - Feb 2008 - Call drops to fast busy

Ok.  The problem that prompted my "best ata" question is this:

I have a person connecting to our asterisk box remotely with a generic 
ATA.  It was actually purchased from Tiger Netcom and is based on an 
HTTEL chipset.

This person says that sometimes they will be in the middle of a call and 
it will drop and go straight to a fast busy.  This is not something I've 
encountered with anyone else.  So is this a:

1) Network problem
2) Asterisk problem
3) Defective ATA
4) ????