How many different frequencies are used in TouchTone dialing?

Answer 1

The TouchTone keypad shown in the image is one used on a telephone from the military AUTOVON telephone system, the keys in the rightmost column indicate the priority of the call being made. These keys are marked from the top down:

1. FO - Flash Override, highest priority
2. F - Flash
3. I - Immediate
4. P - Priority

If none of these keys were pressed before dialing the call was considered Routine, lowest priority.

Any call of higher priority if no available circuits could be found would cause disconnection of lower priority calls (starting with Routine calls and working upward) to create an available circuit for the call.

Answer 2

The touch tones used on phones is known as Dual Tone Multi Frequency (DTMF).
On a normal keypad, with four rows and three columns, of buttons there are only seven frequencies used.
Each column is assigned a frequency and each row is assigned a frequency per row.
Hence only seven frequencies used (3 + 4)
When you push a button two tones are played. One frequency for the column and the other for the row.
The frequencies are designed not to be confused with other, more natural, occuring sounds. The accuracy of the frequencies has to be + or - 1.8% for the exchange to recognise the signal properly.
Some special use phones (military) have an extra column on the right with the letters A,B,C,D. These of course have another single frequency for that column, bringing the total number of tones to 8.

The frequencies are arranged like this

1209 1336 1477 1633Hz
697 1 2 3 A
770 4 5 6 B
852 7 8 9 C
941 * 0 # D

For example pressing the '5' button will combine 1336hz with 770hz, playing a 2 note chord.