Hello
The circuits inside the telephones are simple . did you ever tried to do on off the telephone line ( short and cut ) . you will see that with this action you can get some numbers . it is the basic of all of the telephones . but you can find some other circuits inside the telephone , these days . for example auto pulser circuits , and voice processing Circuit ( todays , that circuit is in simple IC . ) ringer and the other circuits . so , do you want build a telephone with your self ? if yes , i should say that it is simple really , and i built a simple telephone with just one IC and some transistors , 4 years ago . if you want , i can attach here , it's circuit .
However you can design it simply , ( with a clock pulse oscillator and some transistors and a simple keypad)
Good luck
Goldsmith