Standard phraseology says that below transition level, the controller may use the phrase "altitude", or omit it completely, but it is still clear - eg/
BigJet 456, Climb and maintain 5000 feet
or
BigJet 456, Climb and maintain altitude 5000 feet
Of course, "altitude" or the use thereof implies that the controller has communicated (and the pilot has readback) the QNH.
Once above transition level, the phraseology calls for flight levels, and this is always using the standard pressure setting 1013 hPa / 29.92 in Hg
BigJet 456, Climb and maintain flight level 300
Examples inspired by ICAO standard phrasology, however it is important to remember that controllers and pilots quite often stray from exact standard phrasology.