You don't need to be a lawyer to figure out there is no requirement to wear a headset at anytime.
You don't need to be a pilot to read the regs
:
FAR 121.359 (g):
(g) For those aircraft equipped to record the uninterrupted audio signals received by a boom or a mask microphone, the flight crewmembers are required to use the boom microphone below 18,000 feet mean sea level. No person may operate a large turbine engine powered airplane or a large pressurized airplane with four reciprocating engines manufactured after October 11, 1991, or on which a cockpit voice recorder has been installed after October 11, 1991, unless it is equipped to record the uninterrupted audio signal received by a boom or mask microphone in accordance with § 25.1457(c)(5) of this chapter.
14 CFR 121.359 - Cockpit voice recorders. | LII / Legal Information Institute
Maybe your boom mike doesn't have a headset attached.
These are the U.S. requirements, they often filter down to other countries over time.
Now the selection of speaker and hand mic, above whatever, seems to be favoured by that group of F/Os who like to appear "cool" whilst reading the paper/Ipad or whatever THEY think is their function in the upper levels.
Yep, I think I commented earlier on this thread that many FO's try to look like they are doing everything but flying the plane while they are in the seat. I guess it's a generational thing.
