I can not call him because he is in the conference room. (That is, he is busy in a conference and does not wish to be disturbed.)
It is unlikely that a conference room has no phone, or that a person does not have a mobile phone. Now "I can't call him as he is on an airplane" would mean that he can't be reached strictly because of where he is.
It is correct, but you are just using the expression "behind the wheel" to replace "driving." The meaning is the same. It's not his physical location that prevents him from using the phone. It is the activity he is doing.