It means that the sound engineer should use a piece of electronic equipment, a "bandpass filter," to modify the sound. Telephones restrict the range of frequencies they transmit. They only transmit 300-3,000 Hz. Voices heard over telephones have a characteristic, distorted, hollow sound. It is intelligible and easy to understand, but it is not high fidelity. On a movie soundtrack, when people are speaking on a telephone, the sound engineer will pass the microphone signal through a 300-3,000 Hz. bandpass filter to imitate the sound of a phone call. When you hear the soundtrack, you feel that you are hearing voices on a telephone, not voices in the same room as you.

I believe it means that the actor's voice should be muffled (filtered), to imitate the sound of a voice coming from a telephone. See definitions 10 and 11 here: https://www.dictionary.com/browse/filter

It actually means that the actors voice should be put through a special effect filter, so that they sound like. a) a sobbing voice b) a cheery voice c)a telephone operators voice. it is listing each of the kind of voices for the actors.