A high-pass filter, as the name suggests, is one that only lets through the highest frequencies of a sound. It removes all the frequencies below a user-specified point in the spectrum. To recreate a telephone tone, the filter should ideally be set at around 300Hz, so only frequencies above that pitch are audible. This removes a lot of the "richness" from the voice that normally makes it sound round and full, resulting in a sound that is closer to the thin, reedy tone of a voice on the telephone.
A low-pass filter is the polar opposite of a high-pass. It cuts frequencies at the top of the tonal spectrum, allowing only those below the chosen frequency to be heard. Frequencies at the upper end of the spectrum add a great deal of detail and "shimmer" to a sound. Cutting off anything above 7500 to 8000Hz will dull the sound and remove some of that fine detail, such as the sound of the subject's teeth and lips, that can often be heard when recorded with a high-quality microphone.
Piezo transducers have a particular "nasal" quality caused by a spike in their frequency responses. Piezos are particularly efficient when reverberating at certain frequencies, making them boost particular tones. Create a 2-to-3db spike in your equalizer using a narrow band-pass filter. Roll the filter frequency up to around 1500Hz to recreate that distinctive "nasal" sound.
Though not technically a filter, altering the bit-rate or resolution of your audio file can help to produce the desired results. Telephones are distinctly low-resolution; even high-quality units carry a degree of crackling and distortion. Use a bit-crusher or turn down the resolution of a recorded sound. 16-bit is probably as low as you can go without making the sound unintelligible.