Some time ago I developed this modules as add-on for cheap comercial radios. The modules were programmed each with different tones and then inserted in the radios in order to distinguish the talkers.
I remark the tones are DTMF tones. This tones are used to dial numbers on touch-tone phones. In my applicattion, I use the prefix "A" then two digits. So every time one radio is used, in the decoder you can see "A21, A45, A12".
The board size is small enough to fit the mic unit and most of times inside a portble radio.
This module was developed for active_low PTT signal input, and active_high PTT signal output.
Basically, you signal the PTT to the module. The module do the TX, insert the dtmf tones and wait for you to talk and release the PTT. Then the module insert again the tones before releasing the transmitter.
Operates on 3.3V or 5V.
This is a simple project using AT90S2313 and Bascom, SMT Board done in TraxMaker.
The signal level was enough for me with the 100K output resistor, without amplifier.
If you are planning to build a group of modules you need to compile every file with a different ID. That's the boring part as I have not yet included jumpers to setup the tones.
Note: This circuit does not use a crystal but a 4 Mhz 3 lead resonator (w/caps included).
I have developed also a simple Dtmf decoder with LCD and RS-232 output as add-on for the base station receiver. Of course you can use any DTMF decoder including that some VHF/UHF radios have the decoder built-in.
Maybe I will post later the decoder part.