http://www.techlib.com/electronics/telephone.html http://home.att.net/~theremin1/Circuit_Library/line_in_use.htm http://www.eidusa.com/Electronics_Kits_BUSYER.htm http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#inuse.gif http://www.aaroncake.net/circuits/phonbusy.htm http://www.electronics-lab.com/projects/telephone/001/index_en.html Stb. :) Topy