It's possible they may talk in secret, with only their trainers knowing about it, remember that we are only seeing the Pokemon world through the point-of-view of Ash and his friends, not seeing everything else that's going behind the scenes. It's clearly implied that there are other talking Pokemon in existence, ones that DON'T use telepathy like the Pokemon from the movies (especially since I see telepathy as the cheap way out of the talking animal trope).

One thing I've noticed is how in older seasons, people were always shocked and surprised about Meowth being able to speak, but in later seasons, almost nobody reacts to this and they just accept the fact he's a talking Meowth. Either this implies that they've seen this situation occur before with another Pokemon, or the writers are just lazy, most likely the latter.