IFATCA Policy is:

To reduce the possibility of call sign confusion:

• Call signs that correspond to the last two designators of both ICAO and/or IATA airport designators of either the point of origin or destination of the flight, shall not be used.
• In alphanumeric call signs, aircraft operator designators shall not be chosen that will create confusion with phonetic letters.
• The use of tools that reduce possible call sign similarity shall be encouraged.