If you have an older telephone (pretty much any phone with a rotary dial ), then it probably uses a carbon microphone.   Carbon mikes have been around a long time and they were used in telephones because they put out a very large signal, though they're decidedly low-fi.  The earliest telephone circuits included basically a battery, two carbon mikes and two earpiece speakers, all connected in series. The carbon microphone generated enough signal that even without an amplifier it could drive the earpiece.  A carbon mike consists of a diaphragm that compresses a small canister full of carbon granules. When compressed, the resistance is less, and vice-versa.

This was the sort of mike they used in early aircraft, because they were sturdy and reliable, along with fairly high impedance telephone-type earpieces. They also had available the sorts of plugs and jacks available in telephone exchanges.  This set a standard that has not changed in the last 60 years.  Our aviation headsets still use telephone switchboard plugs, and the microphones and speakers are still required to have the same characteristics as the old carbon microphones!

But, I hear you say... "We use electret (or dynamic) microphones in our headsets!!"

Of course, you are right, but if you were to take apart the electret microphone element at the end of your headset boom, you would discover that as well as an electret microphone, there is a small circuit board with a handful of components.  This board is a preamplifier, and it's required to boost the signal available from the electret microphone to the same level as you would expect from a carbon microphone.  It adds nothing, in general, to the fidelity of the signal (Actually it distorts it slightly). It's only there to maintain the standard!

This is the reason that GA microphones cost so much compared with a simple electret microphone element. Of course, the electret capsule used in aircraft is generally a noise-cancelling or directional type, and this adds slightly to the cost as well. 

Air band radios and intercoms are of course designed to the GA standard, and require the microphones to look electrically like a carbon mike, and for the earpieces to have a high (about 150-300 ohms) impedance as well.  They present about 9 volts at the microphone terminal to drive the preamp and mike.

Of course, no such standard exists for non-GA equipment, so most manufacturers use the simplest system they can - generally a bare electret or dynamic mike (no preamp) and commonly available 8 ohm earpieces, for UHF-CB or 2m FM radios.  They also use a huge variety of different schemes for transceiver activation.  Most of them expect any connected electret mike to have about a 2.2k impedance and present about 5V to drive it.  If you connect a GA mike to these transceivers, depending on the type of pre-amp in it, it will either not work at all because insufficient voltage is available, or overmodulate the transmit signal because the output is way too high.   If you try to use a bare electret mike element in a GA application, it will generally undermodulate due to lack of signal.

If your headset uses a "dynamic" microphone, a preamplifier is also required for GA operation and will be found in the earcup.

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