Although a hold feature is standard on most new phones, a lot of us still use the origional bell phones. Those of us that require a hold feature will find this circuit very useful. It is easy to build, and is compact enough to be installed inside the phone with no real problem. It is also powered by the phone line itself, eliminating the need for batteries.
1.5K 1/4 W Resistor
1K 1/4W Resistor
1N4002 Silicon Diode
1N4003, 1N4004, 1N4005, 1N4006, 1N4007
Green LED, Yellow LED
Normally Open Push-Button Switch
Wire, Board, Case (If Used)
1. To place a call on hold, simply hold down the button while hanging up the phone. To take a call off hold, just pick up the phone, or any extension.
2. Even though this is a simple circuit, you may have to check with your phone company before use.
Source: Electronics Lab