that is poor reasoning.
even if you are right about phone not being there when you left the room, theres no ground to say it was ghosts. read about Ockham's razor
http://en.wikipedia.org/wiki/Occam%27s_razor .
You have more reason to believe it was your brother making a joke, or your mom, or a wild monkey that climbed through your window, than inventing some new idea ie. a ghost, to explain it.
heres an applicable excerpt from Ockam:
“nothing ought to be posited without a reason given, unless it is
self-evident (literally, known through itself) or known by experience..."
btw I'm not saying i believe ghosts dont exist, but yours is not a good reason to suppose they do.