I would say that it is legal, as long as you know the person on the other end is qualified and are completely sure that no unqualified person (eavesdropper or cowan) is listening.
So, if you are 100% of his voice, have checked both phones for bugs and, if on a cell, are certain no one can hear you, you are good to go.
Surely the conspiracy theorists aren't our tapping lines to verify what they see in Duncans!
Sorry, I just wanted to lighten up the discussion. I'm sure there are those out there who would feel that talking over esoteric stuff by phone is bad practice, but as I was learning my work there were several times that I called to verify I was right on a line. Still to this day I call him if I am unsure of a line in a degree.
It is mouth to ear (in a sense).