"Mechanical Hold" was a primitive technology for putting a call on hold by putting a short circuit (actually a resistor) across the phone line, when a multiple-line phone was switched to another line.
The holding resistor is in each phone, rather than in common equipment that could be controlled by each phone.
The most common mechanical-hold phones were the #575 (rotary) and 2575 (touch-tone) two-line phones. A rotating knob selected the line, and a call was put on hold by pulling up the left-hand hookswitch button.
Three-line mechanical-hold phones were made by GTE and ITT and possibly others. They looked very much like the standard six-button phones, but had a red hold button and a clear line button for each of three lines. A recall button was mounted above the dial to provide a new dial tone for a second call without physically hanging up.
The ITT 576 and 2576 used a common 25-pair Amphenol-type plug, but the cable had fewer wires.
No control unit ("KSU") was used, but a common power supply could be used to activate the in-use lights.
The big PITA disadvantage of mechanical-hold phones is that only the phone that is used to put a call on hold, can be used to take it off hold.
The phones have neon lights to indicate ringing, that were powered by the phone lines. Normally each phone has one ringer that would ring on just one line. ITT offered a "common audible signal" that worked for up to three lines.