It won't fit on a 500 but it should fit on any standard German phone, e.g. W48, 611, etc.
Here's what'up with the 16kHz tone:
German payphones (and in a number of other countries) and private lines' "meters," worked with "meter pulses." I'm going to describe this using American coinage and fictitious calling rates because that'll still be understandable.
At home you could have a little box next to your phone with something in it that looks like an odometer. It shows how many units you're using when you're on the phone. Let's say each unit is worth 5-cents. Now you make a local call which is charged at 5-cents for every 5-minutes. OK, so once every 5 minutes, the CO switch sends a little 16 kHz pulse down your line. You don't hear it but the meter does, and it clicks forward another unit, and you see you've just spent five cents.
Now you make a long distance call. Let's say the cost of that call is 15-cents per minute. Now at the start of each minute, the CO sends three pulses of 16 kHz tone down the line, so your meter clicks over three units at the start of every minute. This let you know how much you were spending, in real time, so you wouldn't get a surprise when the phone bill came.
Now let's go to a payphone in one of those countries in the 60s or 70s.
The front of the payphone has one or more sloping channels inside, below the coin slots, that are visible through a thick glass or plastic panel on the front. You insert your coins into the slot(s) to dial any call other than Fire/Police/EMT, and the coins rest in the channels where you can see them. Let's say you put in 50-cents in 10-cent pieces, and the rate for a long distance call is 20 cents per minute.
Now when the called party answers, the CO sends four 16-kHz pulses for 20-cents. The payphone interprets the signals as signifying 20-cents, and knows there are coins in the 10-cent piece channel, so it tries to "eat" two coins. Since there are five coins there to start from, you're looking at the front of the phone and see that two of your 10-cent coins have just dropped out of the channel and you have three left.
Now at the beginning of the second minute, the same thing happens again. And the phone knows there is only one coin left so it beeps at you (and maybe a light flashes) to let you know to put more coins in. You put in three more 10-cent coins, and now you see that four coins are in the channel.
At the beginning of the third minute, the CO sends four more 16K tones and eats two more coins. Now you have two coins left in the channel. But you wrap up your call and hang up before the end of the third minute. The phone returns your two remaining coins.
It's a very clever and cool system that I never had the chance to see in person. A few of the countries it was used in were Germany, Switzerland, Denmark, presumably Sweden, and also in Spain.
It would have been excellent to have in the US except that it might have become a target for vandalism, a problem that is much less in the countries where it was used.
There is an older variant of this involving a 60-Hz AC signal sent from one side of the phone line to ground. With that version, you hear the 60-Hz signal, which is mildly obnoxious, but with the 16kHz version you don't hear it because the telephone receiver can't reproduce sounds that are so high in pitch.
If you have any KTAS phones from Denmark, you may find one of those meters built right into the phone, under a little chromed rectangular frame on the top surface of the phone.
In England, the GPO offered something similar for homes and offices, but it looked more like a clockface. Probably France did something like that as well.
However neither England nor France had those types of payphones. There was a time when France used "POA" (Pay On Answer) payphones that were externally very similar to those in England during the 60s - 70s, and so far as I know they were operated by meter pulses as well (darn it, I have three UK payphones here, and the tech info, I should know more about this). However the UK and France payphones used an audible tone ("pay tones" or "rapid pips") to alert callers that they needed to stuff more coins into the slot to continue the conversation.
The closest we got to that in the USA was "dial tone first" payphones typically AE, where you would have a coin ready to drop through the slot the moment your called party answered. Line polarity on Strowger switches would reverse upon answer, causing a relay in the payphone to short out the transmitter until you dropped a coin in the slot. I'm not certain how they handled it after the first three minutes, and I've used those when I was a kid so I should have remembered it. For long distance calls, the operator would ask for the deposit of however-much it was, before letting you speak to the called party.
