Hey artel, what recording program do you use? I use Adobe Audition 3 and I know that I can highlight the part I want to add an effect to and click on the effects tab. It'll have a drop down and I can use one of the equalizers to change it. I know Adobe Soundbooth CS5 (I have that too) has an effect labeled as telephone. I'm sure Audacity has that too, in either effects if they have that or tools. I've never used Audacity :/ You could probably lower the volume of the "on the other end of the line" lines maybe lower the bass and up the treble.
Worse comes to worse, you could step away from the mic and mumble something. Not sure if any of this helps. Sorry man.
Incase you have Audacity I found a little guide thing. I hope that helps man.