hi,
Listen firstly explain what u want to do with telephone interface like r u planning to design a PBX ro some kind of rely based system ...

To interface a telephone with microcontroller you need to firstly know following things:-

1) Telephone Set require minimumm of 12 Volts , 24 Volts to operate and controller require 5 Volts so first thing is to supre impose these.

2)so firstly u need to have a SLIC circuit or SLIC IC to separate 5 volts and 24 Volts.

3) Next thing for DTMF the best is MT3271 coz it can be connected with microcontroller serially so less pins are used.

4) further thing i can guide u if u can explain your project..

and rem this is the forum on the net which can provide maximum help there is no source on the earth i mean on internet from where u can get help on telephony interface with controller