We have been told several times that for an observer any object will seem to move slower and slower the closer it comes the event horizon. In the end, it will seem as if it is standing completely still.
But I assume we are just talking about the information available for us. After all, it would be impossible for a black hole to grow in size if objects never crossed the event horizon.
So even if it looks like an object has stopped moving for an outside observer, how much time will actually have passed outside the black hole’s gravity field when an object has finally crossed the event horizon?
If the magic telephone works this way (it’s magic but still obey to physics when it comes to time dilation), then :
When they approach the event horizon the astronauts will talk slower and slower until they just froze in place.
So it won’t be “much slower” it’ll be “frozen and in place (almost) forever”
From the astronauts point of view tho, they’ll see us get old and die in a few milliseconds.
But at this point we don’t need a magic telephone: this is what would happen with a normal telescope as well.
(To give you another image let say in a galaxy there’s one super nova each century. When the astronaut is going to get close to the event horizon when he’ll watch the closest galaxy he’ll see it sparkle like a Christmas tree with dozens of super nova each seconds.)