The existing BEETEL COREL was using the UM91214A dialer chip which both scans the keypad and generates the DTMF tones.

Click here for spec sheet of UM91214

UM91214 was replaced by W91312N (Winbond) which can be interfaced to the Microcontroller. The chip costs only Rs. 50. It is connected to the 8051 using an 8255 PPI.

Click here for W91312N data sheet

A51 Source code for the dialer

Following are some BMPs of the waveforms of the DTMF tones generated by W91312

1 2 3 4 5 6 7 8 9

Another alternative to the W91312N is BU8307CS.

Click here for BU8307CS data sheet

Click here for a pin-by-pin comparison b/w UM91214A & BU8307CS