When he was checking

[Kolridg]

Do I need include "he was" in the sentence like this? Also I am not sure about whether it must be "when" or "while". Perhaps "when" works better because it is not a long process checking a contact in the phonebook. However, on the other hand it could still take a minute or two, what can be considered as a prolonged action.

Sentence (personal live example):
He accidentally called him when [he was] checking whether he has his number in the phone book already. (He by mistake hit the call button instead of the info button).

 

[jutfrank]

It's not strictly necessary to include he was, no.