OK BOOMER

7778866{844444777_7446666633_444777_26622244433668}

Since the flag starts with sun{, we know that:

  1. 777 -> s
  2. 88 -> u
  3. 66 -> n

It reminds me of the telephone keypad, where each keypad maps to multiple characters, and you must repeat to select the correct character. However, the 7 keypad is weird, 777 should be r instead of s. Nevertheless, we can guess the flag out:

  • part 1:
    • 8 -> t
    • 44444 -> hi/ih
    • 777 -> s
    • this
  • part 2:
    • 7 -> p
    • 44 -> h
    • 66666 -> no/on
    • 33 -> e
    • phone
  • part 3:
    • 444 -> i
    • 777 -> s
    • is
  • part 4:
    • 2 -> a
    • 66 -> n
    • 222 -> c
    • 444 -> i
    • 33 -> e
    • 66 -> n
    • 8 -> t

Flag: sun{this_phone_is_ancient}.