The attendant call simply energizes the 2T wire in the mu system. As previously stated, this is the alarm circuit, so it only works assuming the alarm bell isn't already ringing for some other reason.

There were other uses for the communicating signals even outside a passenger train. I believe they were also used when pushing snow plows and at times when double heading. For instance - three short means back up, but if you're already moving it means slow down (may vary by railroad).

Before radios, the brake pipe was also unofficially saw limited use for communications. For instance, if a train whistled off to depart and the flagman wasn't back on board or some other reason prevented departure, a brake application initiated from the rear brought things to a halt. Likewise a brake application from the rear on a standing train might be used to get the head end's attention or be a signal that the flagman was back.