Though I'm not sure you'd use this name in a written document ...

Call signs in aviation are derived from several different policies, depending on the type of flight operation being conducted, and depending on whether the caller is in an aircraft or at a ground facility. In most countries, unscheduled general aviation flights identify themselves using the call sign corresponding to the aircraft's registration number (also called N-number in the U.S., or tail number). In this case, the call sign is spoken using the ICAO phonetic alphabet. Aircraft registration numbers internationally follow the pattern of a country prefix, followed by a unique identifier made up of letters and numbers. For example, an aircraft registered as N9876Q conducting a general aviation flight would use the call sign November niner eight seven six Quebec.
In most countries the aircraft call sign or "tail number" or registration marks are linked to the international radio call sign allocation table, and follow a convention that aircraft radio stations (and by extension the aircraft itself) receive call signs consisting of 5 letters. For example all British civil aircraft have five-letter call signs beginning with G. Canadian aircraft have signs beginning with C-F or C-G, such as C-FABC.
http://en.wikipedia.org/wiki/Call_sign


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Note added at 6 hrs (2007-11-16 23:21:28 GMT)
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Found ghits that DO use "callsign" as a written ID number.