Re: Long ago.
We had a new IBM AS/400 that had 16 Fax cards installed to receive faxes with Credit card applications and file them automatically for processing. All of our branches had Xerox fax machines installed. We had issues with faxes not receiving correctly. IBM assured that they had tested the system with the particular model of Fax machine we used and it worked fine so the problem must be with the setup of the Fax machines.
We had an Xerox engineer come on site and I watched as he whistled down the line to get he fax machines and fax cards to negotiate the connection. He was even able to listen and tell if the machines were responding correctly.
In the end we determined that the issue was that IBM had tested with US homologated fax machines and we were using Australian model fax machines. There was a difference in the levels output by the Australian fax machines so the IBM engineer bought in a jury-rigged attenuator to reduce the levels on the line and the fax machines worked properly.
So from first hand experience I can assure you that some people could indeed whistle down the phone line to get fax machines and modems to respond.
Biting the hand that feeds IT
