Why is there no "q" and "z" in dtmf dial?

Question

Why is there no "q" and "z" in dtmf dial?

Answer 1

A very small number of POTS telephone sets were manufactured with the letters "Q" and "Z" shown on the "1" digit, but this must not be considered to be a standard assignment. Many cellular phones put "Q" on the "7" digit button and "Z" on the "9" digit button, but this differs from wired telephones too. All of this means that the handling of "Q" and "Z" are not consistent, and "named" telephone numbers can't be used predictably if they contain "Q" or "Z".

Answer 2

The answer lies in the original purpose of lettered dials.   In many places the first one, two, or three digits of a phone number were dialed as letters which represented the exchange name.  In the early days there were many regional variations in the dials (rotary at that time, of course), often localized to one city.   Letters were assigned to number positions to suit whatever naming scheme was to be used, so to take a hypothetical example if a town was served by four central offices which were named Central, North, Riverside, and Western, they might have just assigned the letters C, N, R, and W to four of the digits so that callers could dial C-1234, R-8704, etc.

To take a real example, at one time dials in Tampa, Fla. had letters assigned as follows:

1 -

2 F

3 N

4 B

5 Y

6 W

7 M

8 H

9 S

0 -

Gradually these local variations gave way to the standard dial with which we are familiar today (2-ABC, 3-DEF, etc.), although a couple of places retained their regional dials right through to the 1940s when interstate toll dialing made a standardized scheme essential to avoid confusion.

It's because the letters were used for exchange names that Q and Z were considered unnecessary, since they are of little value and the remaining 24 letters fit neatly 3 each onto the digits 2 through 9.  

Right up until the 1960s, and even later in a few cities, these letters were still being used in telephone numbers, e.g. MAdison 2-1234, ACademy 4-9876, PLaza 1-0234.

Some rotary dials back in the 1940s did actually add a Z to the zero position, although it was never used for dialing an exchange name.    It's unclear exactly why it was added, although I've seen a couple of related theories.  One is that it was to catch people told to "dial zero for operator" who actually looked for the letters Z-E-R-O to dial instead of just "0."  The first Z would be 0 anyway, and thus go to the operator.

There were also Zenith numbers in many areas, which were the equivalent of 800 numbers in their day for large businesses.   You could not dial them directly though, but had to call the operator and just ask for "Zenith 1234" or whatever the number happened to be.  By placing the Z on the zero position anyone who tried to dial Zenith 1234 like a regular number would dial "0" first and reach the operator anyway.  

Z on the zero position disappeared again when the Western Electric 500 telephone sets were introduced during the 1950s, and the basic 2-ABC, 3-DEF, 4-GHI, etc. pattern then continued onto DTMF keypads from the 1960s onward, with zero just being marked for "Operator."

In other countries letter arrangements could be quite different to allow for their own place names used for exchanges.  

British dials evolved to almost the same standard as American, except that we put the letter O on the zero, leaving just M and N on the 6 position.

Over in France they had dials with the same arrangement of letters on 2 through 9 as the British dial, but because they had a couple of exchange names in Paris which included the letter Q, French dials had both O and Q on the zero position.

Some British dials in the 1960s were also made with a Q on the zero position to allow for this when direct overseas dialing to Paris became available.

In fact international dialing and the differences between national standards for the letters was one of the reasons that letters were phased out in telephone numbers.   Even between the U.S. and the U.K. where dials are very similar, there existed plenty of scope for confusion on a London number such as ACOrn 1234 (220-1234 vs. 226-1234).