A quick grammar question...

[Paris]

Mike couldn't answer the phone when tony called him because he was working in the lab.

Can I also say:

Mike couldn't answer the phone when tony called him because he had been working in the lab.

If I cann't say the sentence in this way, why?
Thank you :roll:

[Mister Micawber]

.
It doesn't make much sense with the past perfect, because the lab work and the phone call presumably happened at the same time.
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Native English teacher at Mister Micawber's