You know how to press it but do you know how it works?

Before we move on to the theory behind this, just take a look at these sound clips.

 GOTO clip1: http://www.genave.com/audio/dtmf-page-normal-S00P0501P99.wav                                                                     
GOTO clip2: http://www.genave.com/audio/dtmf-page-slow-CP4D34SS.wav

DTMF- Dual Tone Multi Frequency

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

Did you ever think of how your response (pressing a particular number) is identified by the receiving side? For an example let’s consider a situation where you are in an Interactive Voice Recording (i.e. calling to customer care).  They asked you to press 1 or 2 or any number to identify your response. Pressing a key will send a sinusoidal tone for each of the two frequencies (i.e. 697 and 1209Hz for “1”). See the image below. The multiple tones are the reason for calling the system multi frequency and it is called as dual tone because of the use of two frequencies. Two frequencies are there to make sure that frequencies generated accidently will not be identified as a press. Columns will have higher frequencies than rows. In normal dialing also we can use DTMF, but only for special purposes (if needed MSC can identify the pressed keys).

Interactive Voice Response (IVR) - is a technology that automates interactions with telephone callers.

Figure 1: DTMF

Pulse dial system was used earlier and it is a slow and older method. For optimum use, push-button phones utilize DTMF signaling, which was also known as touch-tone dialing. 

                                                            Figure 2: Push Button Phone

Now you know the theory, and let’s see how it is implemented

Mark and Space

Mark refers to the duration a DTMF tone is produced (the time which a DTMF digit tone is actually producing sound), and Space refers to the duration of the silence between individual digits.
Notations

• Mark/Space : Pronounced as "Mark and Space”. Refers to the durations of mark and space.

•  40/40: The decoders expect the DTMF tones to exist for at least 40 milliseconds and in silence for 40 milliseconds between each DTMF digit.

References: Some of the images, details and sound clips were taken from the following website. http://www.genave.com