So, a pair of my lucky colleagues are fighting to work out why they can't get a Linux machine to successfully dial up to an external company's machine, which we know is running Windows 2003 server. In an attempt to replicate the problem, they are attempting to replicate success with Windows on both ends inside the office, before moving on to replicating failure with Linux as the dialling machine on a known good internal office setup. This in turn, isn't working...
However, in the process of diagnosing why, he rang the external server, listened to what it chirruped at him, and compared with what the internal line chirruped.
"It's different," he said. "their one goes be-boop first."
This isn't a guard tone, at least not ones that a US Robotics modem can generate (constant 1800Hz or constant 500Hz) so we wondered what special hardware they had.
About 5 minutes later I blurted out "I know what that is. It's the noise the exchange generates for 'this line does not accept reverse charges calls.' It's what you get if you dial a payphone".
Cue rest of the office asking "how do you know that?"
Which was amusing. As actually, they realised that they do know that. It's just that I was the only one to remember it.