Telepathy question

[DizzyKungFu]

I'm confused about how telepathy works. Telepathy from a demon, devil, pseudodragon, or the Mindbender prestige class, let's say.

Does this telepathy allow the subject to respond telepathically, or do they just hear the words in their mind? Can a being with telepathy communicate with someone through a wall, even though they can't otherwise sense the person?

 

[Murrdox]

For Ghostwise Halflings in Forgotten Realms, I believe they get telepathy with one creature within 30' of them, and I think they have to see them.

I'm not sure if this is the way it works for other creatures/classes or not.