From 4444, with exceptions, likely the same everywhere
2.5 ATC route clearances shall always be read back unless
otherwise authorized by the appropriate ATS authority, in which
case they shall be acknowledged in a positive manner.
2.6 All clearances to enter, land on, take off on, cross and
backtrack on the runway-in-use shall be read back.
2.7 Other clearances or instructions, including conditional
clearances, shall be read back or acknowledged in a manner to
clearly indicate that they have been understood and will be
complied with.
2.8 Runway-in-use, altimeter settings, SSR Codes, level
instructions, heading and speed instructions and, where so
required by the appropriate ATS authority, transition levels,
shall always be read back,
For everything else a readback is not mandatory.
That said, it depends by the situation if it's appropriate or not to reply after a "standby", pilots should be able to judge if it's the case where they are asked to shut up or instead just to wait for ( a request they made or such as)