I have an old RCA desk phone (model 25202RE3-B) plugged into an OBI200 that I use with voip.ms. Everthing works fine (message indicators, multiple lines, etc.) but approximately every 30 seconds I hear a faint click from the phone speaker the display back light comes on. It stays on for 13 seconds and then turns off. Not the end of the world, but mildly irritating. Any ideas as to what the cause might be? Doesn't seem to happen if I plug the phone into a POTS line.