Why do some airlines use alphanumeric callsigns in the flightplan when the flight number printed on the ticket consists of only numbers?

For example, the flight TK871 uses callsign THY4JT.


when flight delays cause a duplication of a flight number- there is a suffix added. For example- flight 1423 DFW/OKC is also operating as OKC to DFW. Normally a turn of equipment- however, DFW/OKC delayed significantly- equipment and crew is available to operate 1423 OKC/DFW. These 2 could be in route simultaneously- one is usually designated as a suffix- 1423J Call sign American 1 4 2 3 Juliette

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    $\begingroup$ Welcome to Aviation.SE! Please read the help center to see what makes a good question / answer. Also have a read of other answers on questions for examples of what makes a good answer. Generally well formatted answers with references do best. $\endgroup$ Aug 2 '17 at 20:08
  • $\begingroup$ The question is regarding completely changing the number from 871 to 4JT, not just adding a letter at the end. $\endgroup$
    – fooot
    Aug 2 '17 at 22:22

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