Suppose player "Apple" has telepathy, and tries to communicate with creature "Bagel," whose stat block explicitly says "understands Common but can't speak."

Bagel can't speak, but presumably can understand Common and think in Common. If they can think in Common why might they be incapable of replying to a telepathic message?

Trying to understand how telepathy with a normally mute character or creature might work. I can see it might depend on the class feature or spell that enables the telepathy, so if that matters we can narrow to the telepathic speech feature of the Aberrant Mind Sorcerer.