Pretty sloppy design Gil. For R1 the series resistor with a Zener diode with value 6.2 volt, he advices a value that sets the zener diode "dry " at 15,3 mA (not MA because that means Megaampere).
According to the spec sheet of this Touch Tone Decoder, ( http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS006975.PDF
) the used current without the load (yellow-LED) current is typical without and with lock between 8 and 14 mA (milliampere) Add the current of he load of the yellow LED estimated 10 mA, and you can watch that "typical" during lock (burning LED) the Zener stands dry , and hence the Vcc voltage is not 6,2 V and hence al the parameters are different.
All stuff of no code technician licence requirements, Isn't it?
Who cares. We are all sucking hams, aren't we? Only buying equipment and not anymore able like our predecessors, to create their own equipment from parts, just around in the junk box.
The developer of the circuit probably explains "It works here, I marketed a kit, and if it doesn't work at your location after assembling, on which fact you can be very proud, because only the most stupid guys buy kits, and so I never ever get a complaint, because they fail to assemble it, you must have made a stupid error." "Stupid questions do not exist, so your question does not exist, All sat" "The overpaid components in the kit, with which I paid may luxury world wide cruise vacation, are factory guaranteed. So contact Intel, Vishay and so on to file your complaints. You must be a pretty dumbo, a typical kit-buyer, kind of after 40 years retired Morse number station copyer, I suppose; so shut up please instead of wasting my valuable time" .
So not my words, I am a friendly Dutch guy, all Dutch guys are friendly, the most friendly people you ever can meet, but it are the supposed thoughts of the United States kit marketeer.
When you are still reading, and want some additional comments about lock-time and bandwidth of the touch tone decoder, and miss the brains to find out for yourself from the refered specsheet, I am at your service, all for free for the asking.
And Oh yes, excuse my crippled English, not my mother tongue . Dahdah stays for milk but also for melk. When you are a typical proud kit builder just as K8AXW, reading and understanding my message may require a lot of ham spirit as he said before in the forum.
So better annoy it, like K0CBA says he does, and ask other guys around here what the meaning is, saves time.