Audiotent Tip 375. Telephone Vocal Effect.
Here is a little trick to create a lo-fi vocal effect using just an equaliser.
First, insert your favourite eq. Flexible digital eq would work best for this technique.
Next, high pass the low frequencies to around 300-600Hz and Low pass the high frequencies above 4-7kHz.
This will now limit the frequency response, making the vocal thinner, more lo-fi, replicating how it would sound playing through a small telephone speaker.
Finally, add a few (2 or 3) narrow boosts (high Q value) in random frequencies from 600Hz to 4kHz. This will replicate some resonances and add even more character to the effect.