Converting speech audio to telephone audio
Band-pass filtering with cut-off frequencies of 300 Hz and 3400 Hz should result in a good approximation. Try with a Chebychev filter or order not more than 6.

Then you may need to downsample your audio to 8000 samples per second, which is the standard for telephony.

P.S. The actual cut-off frequencies (especially the 3400 Hz) may be different according to different sources. I'll edit this answer when I find the official ITU recommendation.