So, a question I get asked weekly and one that is so simple to answer that I might even leave this page blank...hahaha! 

If you read my tutorial on Filters then you are almost 80% there.

To create the 'telephone' effect you need to use a filter/EQ and narrow band the frequency range by rolling off both the low and high end to leave the mid range band. This is narrow banding a frequency range.

Basically, you are squeezing the vocals into a narrow part of the frequency spectrum (narrow banding). 

By using steep filters it is easy to narrow band the frequency range to between 300Hz and 3kHz.
This, of course, depends on the tonal content of the vocals you are trying to effect. Judge to taste so to speak. The range I have provided is not a given template for all vocals but a guide as to what constitutes the range for mid tone filtering.

In terms of filtering you only need to use a banded EQ (one that provides bands for all frequencies), although a sensible range of bands will suffice.
What you are trying to achieve, in street talk, is to remove the low end and the high end so as to be left with the mid range.

One of the best tools for exactly this process is the band-pass filter as the band-pass filter passes frequencies between two limits. It attenuates frequencies below and above the cut-off and leaves the frequencies at the cut-off.

If you cannot be bothered to use EQ or filters then there are many plug-ins that will have a 'telephone' effect preset amongst their list of pre-determined templates.

So long as you stick to the concept of narrow banding the frequency range you should be able to achieve the telephone effect with ease.

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Eddie Bazil (Zukan)
www.samplecraze.com