I used to repair fax machines...
...for Sharp Electronics UK.
Bearing in mind this was the -very- early 90's and home faxes were still new and to some folks, genuinely space-age.
I genuinely had to speak to customers (often of a more elderly persuasion) to explain one of two things: no, it really has sent it, even if you have the original in your hand (they honestly believed it somehow sent the original like a letter! Or (far more commonly) no, there is nothing wrong with either fax - try turning the one you're sending over...when they called to complain it was just blank at the receiving end.
You used to be able to put them into a diagnostics mode as well and one of the tests was the fax tones. I was so practiced, I could tell a duff modem chip just by the slight change in the warble.
Oh and then there was the time the MD brought a box in, plonked it on my desk and told me he'd told his mate I could fix his fax machine for him...he'd dropped it down several flights of concrete stairs.
The "repaired" one he got was made mostly of scavenged parts from previously scrapped units.