samson. I had to apply for a callsign once. It was a nightmare. Ring AIS at Heathrow airport tower building. The number is 02087453419 the name is Cyril. He will tell you the requirements. Basically you must fax him a request of your callsign and three letter code which goes on the flight plan etc. This is sent to Canada where they approve or reject it. He normally can reject it over the phone as most of the callsigns you come up with have all been taken. It took us ages to find one we liked that had gone. There is a ICAO Doc you can by for £30.00 pounds od that has the latest callsigns and three/four letter codes issued world wide. Hope this helps. Oh it must be a company headed fax message to apply.