I don't know of any plugins designed for the "telephone" effect, but in a pinch you can replicate it with a parametric eq. The idea is to harshly remove the lows and harshly remove the highs leaving only a honky midrange. Here's how to do it:
Take your lowest shelving knob and turn its gain down as far as it will go so that all of the lows are cut. Next, take your highest shelving knob and again turn its volume down as far as it will go in order to cut all of the highs.
Next, take a midrange frequency knob (turn down your speakers first!), and slowly crank the gain up as high as it will go. Start with the midrange frequency at around 1k or 2k to start.
Next, take your low shelf frequency and set it to about 800 or 900Hz. Take your high shelf frequency and set it at around 2-3KHz.
If you have an eq plugin with a graphic display, the eq shape should resemble a sharply peaking mountain.
The entire idea of this is to kill the highs and lows while leaving a narrow band of midrange.
Experiment with moving the low, mid, and high frequencies around until it starts to sound like what you're looking for. You can also grunge it up just a bit with some distortion or a speaker simulator or something like that.
I hope this makes sense. If you need some clarification, let me know.
