#1
Everything I tried for the title was too long so I decided for this. How to mix your vocal to make it sound like it's sung through a intercom, or and old mic, or just filtered? What freqs do I need to add, what do I need to cut?

Like this

http://www.youtube.com/watch?v=VXKFXVjFhuU

Or this

http://www.youtube.com/watch?v=w47RNPRe3co

Or this

http://www.youtube.com/watch?v=39cZelwLKqk&feature=related

I'd appreciate your help!
#2
play around with Cutting the Low and High ends. Cut them to your desired level.
Should work, I've heard adding a touch of distortion spices it up a little
#3
Telephone mids are what you need my friend. Boost 1.2K with a wide Q, add some saturation (for example Massey Tapehead Medium) and have vigorous HP/LP settings.