I needed to do a quickie telephone effect to someone's voice in FCP and found this way was quick, simple and contained within FCP:
1. Apply the AUHipass filter to the audio clip. It's in Audio Filters > Apple
2. Set the cutoff frequency to between 1900 to 2000 and the resonance to about 18.
Keep in mind that these numbers are what worked for me. Your audio may need drastically different numbers.