According to the MM books, telepathy is described as:
A creature with this ability (supernatural) can communicate telepathically with any other creature within a certain range (specified in the creature's entry, usually 100 feet) that has a language. It is possible to address multiple creatures at once telepathically, although maintaining a telepathic conversation with more than one creature at a time is just as difficult as simultaneously speaking and listening to multiple people at the same time. Source: MM, MM3
If a creature with telepathy knows that someone is within 100ft, can the creature communicate with that person, even if there is no line of effect or sight?
My answer would be yes, because no book says that telepathy is blocked by walls, etc.
Normally, a supernatural ability is a standard action to start. If the creature wanted to communicate for more than one round using telepathy, does the creature need to use up a standard action every round they are communicating or only in the first round?
My answer is only in the first round, since the description says that maintaining telepathy is hard as speaking and listening normally.
If a person the creature is communicating with moves out of the 100ft range, and then moves back into the range. Does the telepathic link need to be established again?
My answer is yes, another standard action is needed to establish the telepathy between the two creatures.
I should add the telepathy is supernatural, and a character is looking at getting it from the Mindbender class (from Complete Arcane). And also get the Mindsight feat (Lords of Madness). I couldn't find anywhere to rule whether a supernatural ability should take a standard action every round it is on.