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

[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.
,
_________________
Native English teacher at Mister Micawber's