Every once in a while you need to come up with a simulated phone call effect. It should be pretty common knowledge how to go about this and if you don't already know it you can look it up and find the answer on literally hundreds of websites.
The short answer is that you just band pass the audio so that everything below 300 Hz and everything above 3.4 kHz is rolled off. Add a little compression and you're pretty much done.
That's only a partial answer though. What most people don't realize is that except for the "last mile" of copper wire into customers' homes, the telephone system has been largely digital since the 1960s. So to really push your phone voice sound effects over the top you need to take some sampling into account.
Digital audio is hard and bandwidth is expensive. So Ma Bell came up with a spec that gives you acceptable audio with a minimum of load on the system. A lowly T1 line can handle hundreds of phone calls if they're all sampled at 8 bits/8 kHz. The simple way to get your audio to sound the same is just record a voice over like you always would and then when you render crank the output settings down about much as you can. In my DAW 8/8 was as far down as it would let me go.
If you have to do it live for a performance you'll have to make do with just EQing. But when you've got the time and the resources, know you know everything you need to know to get it just right. Here's a little mini podcast that says basically what I just said here but with some examples in the middle.
- SNR Mini Podcast - Phone Voice - Jon Dayton explains why you sound the way you do on the phone, with audio examples.