there is no such thing as an Android voicemail and iPhone voicemail; the voicemail voice would be the same no matter what phone / model he had (it’s the carrier). He may have his calls forwarded to a work phone during certain times of the day while at work so he doesn’t have to carry around two phones (just an example). In regards to the phone going to voicemail/no location sometimes - have you ever considered there just may be a bad cell area in his office/job area? this would cause a phone call going straight to voicemail/no location showing.

it definitely seems as though you are looking for reasons to believe something else is going on - if you think about this logically, such as I did above, you wouldn’t think it looked so suspicious. but it definitely seems there may be more going on behind the scenes between you guys.