Mil Phraseology:
ATC: Callsign, Climb and report Level FL 50
A/C: Climb and report level FL50, Callsign
On reaching:
A/C: Callsign, Level FL50.
ATC: Callsign, Roger
Civil Phraseology:
ATC: Callsign, Climb FL50
A/C: Climb FL50, Callsign
No requirement to call level
OR
ATC: Callsign, Climb FL50, report reaching
A/C: Climb FL50, wilco, Callsign.
When level:
A/C: Callsign, maintaining FL50
ATC: Callsign, Roger.
(Note that in Civil Phraseology, although the phrase "Climb and Maintain XXXX" is in common use, it is NOT standard phraseology. "Maintain" should only be used to refer to level flight.)