"Can you explain where and when my entire call sign is required and when it is not?"
- Anthony S.
“On initial contact with ATC, a pilot should state the aircraft type, model or manufacturer’s name, followed by the full digits or letters of the registration number. If you do not use the manufacturer’s name or model, then state November (for US registered aircraft).
After initial contact with ATC is established, the controller may initiate an abbreviated call sign by using the prefix and the last three digits or letters of the aircraft identification. Then, the pilot may use the abbreviated call sign in subsequent contacts with the ATC specialist.
Call signs should never be abbreviated on an initial contact or at any time when other aircraft call signs have similar numbers or sounds.
Improper use of call signs can result in pilots executing a clearance intended for another aircraft. Pilots, therefore, must be certain that aircraft identification is complete and clearly identified before taking action on an ATC clearance.
When aware of similar call signs, ATC specialists will take action to minimize errors by emphasizing certain numbers and letters, by repeating the entire call sign, by repeating the prefix, or by asking pilots to use a different call sign temporarily.
You should use the phrase “VERIFY CLEARANCE FOR (your complete call sign)” if any doubt exists whether the clearance is intended for you or another aircraft.”