Refer to figures.
ICAO Doc 9432 - Manual of Radiotelephony
2.8.3 Issue of clearance and read-back requirements

2.8.3.5 The following shall always be read back:

a) ATC route clearances;

b) clearances and instructions to enter, land on, take off from, hold short of, cross and backtrack on any runway; and

c) runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether issued by the controller or contained in ATIS broadcasts, transition levels.

ICAO Annex 10 Volume II - Aeronautical Telecommunications
5.2.1.4.2 VERIFICATION OF NUMBERS

5.2.1.4.2.1 When it is desired to verify the accurate reception of numbers, the person transmitting the message shall request the person receiving the message to read back the numbers.

Squawk 4321 → SSR code

Note: The term 'readback' refers to the process of repeating clearances and data given by the ATC. The pilot is required to read back the SSR code (Squawk 4321) and the new VHF frequency (131.325).