That quote originally came from Alan Watts, not Terence McKenna (who, incidentally, didn't hang up the phone until near the end of his life, when he supposedly was too scared to do more mushrooms).
Alan Watts said:
"[P]sychedelic experience is only a glimpse of genuine mystical insight, but a glimpse which can be matured and deepened by the various ways of meditation in which drugs are no longer necessary or useful. When you get the message, hang up the phone. For psychedelic drugs are simply instruments, like microscopes, telescopes, and telephones. The biologist does not sit with eye permanently glued to the microscope; he goes away and works on what he has seen."[1]
To which I think it was Ram Dass or Dennis McKenna who answered: "I don't hang up on my teachers."
Also, what if you forget the message (as often the states in question are ineffable and the insights one gets are not able to be carried in to ordinary waking consciousness, or not for long)... or what if there is no message, just as seeing a sunset is devoid of messages but still worth seeing? What if you consider the experience sacred or a way to connect to your god(s), the spirit world, transcendent reality, or nature? What if you use psychedelics to enhance your creativity, have fun, or treat your depression?
None of these uses fit neatly in to the narrow "message" paradigm that Watts was talking about.
[1] - https://medium.com/@allevity/they-killed-alan-watts-quote-8b...