I find it easier to work under the assumption that all the characters are always wearing telephone headsets unless otherwise stated.
It sometimes backfires though. The other day I found myself shouting at a character who was talking to another character "You rude rude bastard, don't you know it's rude to talk to someone while they are engaged in a conversation with someone else via their wireless telephone headset, and what about the poor sap you are ignoring at the other end of your own wireless telephone headset???"
After re-reading the comic a few times I realised that neither of the characters had probably been written as wearing telephone headsets, but my anger was still real and present. It's a fucking minefield to be honest.
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GOVT. Warning: Do not smoke around children they will badger you for fags.