Definitely some fairly heavy EQ'ing - a lot of low cut, possibly some high cut and probably some mid-range boost. Try adding a hint of distortion too. You can use the built in SoFo plugs to do all of this.
Easy way: Grap a copy of Audio Cleaning Lab. It has a preset filter that makes speech sound like telephone conversation along with several other presets plus a graphical real time equalizer.
Hard way, using a equalizer:
Reduce frequencies below 350 to -12db.
The range from 351-600 about +6 db.
From 601-900 about +8 db.
From 901-1100 about +12 db.
From 1101-1300 about +8 db.
From 1301-6000 about +4 db.
From 6001-9500 about -3 db.
From 9501-11000 about -6 db.
over 11000 about -10 to -12 db.