This movie was about robert downey as an architect where he thinks a stranger, who he let use his phone, secretly moves into his house. At the end, robert downey jr. inadvertently does the same thing in his neighbors house. Please, does anyone know the name of this movie. It was on cable TV.

Not sure if this is what you are looking for but it sounds a lot like the Spanish film Habitante incierto, El (2004)

Home movies... nice quinn
Mabear, here is a site with all of his movies listed see if you find something you recognize
Robert Downey Jr.