This question prompted me to consider the difference between 'while' and 'whilst'. I have already scanned the previous questions (here and here ) about this but there is still something not reported (I think).
I can say :
He phoned me every day while he was in prison.
Or I can then say :
He phoned me every day while in prison.
But I find that I am more satisfied with :
He phoned me every day whilst in prison.
Is there an invisible verb within 'whilst' ?