Complex sentence: Specially-modified Boeing 747-200s (Boeing VC-25s) are used to ferry the President of the United States; when one of these aircraft is in use by the President, its call sign is Air Force One.
Split into simpler sentences: Specially-modified Boeing 747-200s (Boeing VC-25s) are used to ferry the President of the United States. When one of these aircraft is in use by the President, its call sign is Air Force One.