Is it correct to say?

[Unregistered 77]

My boss is talking to somebody on his mobile and at the same time I got a call asking for my boss. What should I say?
The boss is busy on his mobile. or
The boss is busy over his mobile.
or something else?

[reader_eng]

I would say " I´m sorry Mr/Ms,but Mr ( you boss´ name ) is on the phone right now"
I think it doesn´t matter which phone it is , does it?

[Tdol]

If you want to specify the mobile, the preposition would be 'on'.