Here's an easy way to get the "telephone" style lo-fi vocal effect for your recordings, simply use a telephone! To make the telephone mic you will need an old telephone handset, an output jack, and some wire.
The first thing you need to do is take apart the handset by unscrewing the speaker and mic caps. Next, remove the mic element, which you can save for another project. Most telephone mics are carbon microphones, which require a power source. This isn't incredibly difficult, but using the speaker in the earpiece as a microphone yields a similar sound with much less effort. All you will need to do now is mount the output jack, wire the output jack to the speaker, and install the speaker as the mouthpiece. Replace the caps, plug it in, and there you go!
Please note that you can easily use the speaker as microphone trick with a lot of different types of speakers. In this case, some of the resonance from the phone handset help to give the lo-fi "telephone voice" sound. This mic will also work on drums and guitars, or anything you are looking to make sound trashy. Have fun and enjoy!