It's been a few years since I've had to tweak a faxmodem card, but I think the clue may be here VVV
I think that the fact it's ADSL may be less relevant than the*sending* number or -- more specifically -- the Chipset of *their* faxmodem and its compatability with yours?
I'd be interested to see if the problem is replicated if these senders sent faxes to one of your other (fault-free) clients' numbers ...
The only ADSL-related aspect I can think of woudl be if the Microfilter is faulty. But you say you've already replaced it.
Locate your Mobile phone: Great gifts: