Keying a radio mike

I have an intercom design I'm working on that is meant to key a mike with one line.

I remember the way this works is that the radio has a pullup resistor and when the microphone is applied the DC voltage is pulled down some and the radio goes into transmit mode. The audio itself is AC coupled off the same wire. There is an external Push-To-Talk button you press to connect the microphone to the radio input.

It's called "land mobile" format. I remember that much. However the trouble here is that I've lost the link to the page or pages I had explaining the voltages and impedance specs involved. And a major problem here, the Department of Defense released a new radio system called "Land Mobile" which unfortunately swamps my search results (many commercial pages) and adding "ohms" or "microphone" doesn't help.

Anybody know that spec or have a link to the appropriate page?

I don't know the spec, but this is a circuit that will do the job. The mic should be low impedance, dynamic type. C2 minimizes the pop that occurs when the switch is closed.

Sorry other way around. I have an intercom design I need to make sure the intercom's output is able to key any radio properly.