This is a thin plate that you glue to the wall to indicate that a telephone wire is behind the plate.  I'm renovating my basement and covering over the ugly telephone jacks (because who uses wired phones any more) but I wanted to make the wires findable for me or future owners so I created this indicator plate.  The plate is only 1mm thick and is designed to be painted over so that it's visible but doesn't stand out.  To glue it to the wall I used construction adhesive but most any glue should work.  The plate also covers up the hole in the wall so you don't need to patch it.