As long as your telephone connection is "wireless", there is not much reason for the world outside the event horizon to look different to the participant who has passed in beyond the horizon. The main difference would be that incoming signals would be blue-shifted because of their gaining energy by moving down the steep gravitational gradient.Another reason for the blue-shift however, is that, because of the intense gravitational field, time would be passing more slowly from the point of view of the observer within the event horizon.
A more urgent concern on the part of the ingoing participant, is that, for as long as he does not hit anything, he will be accelerating faster and faster, and experiencing more and more intense tidal forces, before, during, and after passing the event horizon.
When he does hit a stop, that stop will be a Lulu!
Even though the partner outside will notice nothing special. Nor will he receive any messages from beyond the event horizon.
And if your phone were not wireless, no problem: the wires would break no matter what they were made of, even neutronium.
