flexfax: Remote phone behavior

[Keen Young <kyoung@usinternet.com>]

Hello,
  We send daily invoices to our customers. One of them says that every
time our fax server sends them a fax, the phone on their fax machine
rings. They say it only rings from our fax server, and not when other
people fax them. I s there some sort of setting in Hylafax that would
do this? None of our other customers have mentioned this. Is she just
goofy?
Stumped