As the other poster mentioned, EQ will make it sound mostly like a phone. Roll off the highs at around 4-6 KHz and the lows at around 200 Hz. push 1KHz a little too.
It also helps if you compress the heck out of the dynamic range of the audio, very low threshold and a high compression ratio. Phone systems have next to no dynamic range.
Lastly, to simulate the ‘plastic’ sound of the phone handset itself, try adding a little bit of delay in the 10-15 millisecond range. Not enough to create an actual delay effect, but just a hint of it, to simulate the sound-waves bouncing around off the plastic of the phone itself.
If all that’s too complex, just stick with the EQ…
Cheers
Mark M. Pinder
Chief Technical Engineer
Paperny Films