Aims - to clarify PPR's post, in the UK in both cases set the appropriate setting at the time ie QNH when descending and 1013.2 (29.92) when climbing to a flight level. Do NOT wait until TA. Any required altitude reports on the way up can be given from the standby which can remain on QNH until no longer required.
This system has been standard for a while now and is designed to prevent 'level busts'.