AudioKit is great for sound design. This playground creates canonical telephone sounds.
A dial tone is simply two sine waves at specific frequencies
The ringing sound is also a pair of frequencies that play for 2 seconds,
and repeats every 6 seconds.
The busy signal is similar as well, just a different set of parameters.
All the digits are also just combinations of sine waves
The canonical specification of DTMF Tones:
User Interface Set up