DTMF Tones

hello friends every body uses the cell phones now a days and everybody faces the normal curcumstances like balance cutting and something other.And there is not any problem then also we dial a customer care number for balence enquiry or to enter a recharge voucher. I think every body faces these simple things and call to customer care.
now my question is that when we call to customer care services then our call is transfered to the computer system and it gives us instructions and we have to follow these one to get our required enquiry but when we follow the instructions it said that press numbers for different enquires and we press them on our phone and we get the required outputs. But i want to know that we presses the number on our cell phones then how can that computer system know that which key we have pressed and how can it gives us the required enquiry. I know a thing that when we call to a number then a binary electronic signal is generated and we will alloated a specific chat room for our caller and us and we will able to talk to him easily but in this key input features i think there is something other thing which works and i want to know which procedure is responsible for it and it based on which principle or how can it recognise the input send by us.
so pls somebody tell me the required information for this thing.
this platform is very good for these informations and i will very greatfull to the admin who provides us such a great platform for us.