This is certainly unusual. Does this occur when you talk to others when not on the telephone? Does this occur during the day and/or night? Does this occur only in one particular location?
sounds like a nervous habit. Similar to a twitch.
It only happens when I talk on the phone, not in normal conversation and I don't think it is restricted to any particular phone or location. I suppose it could be nervous twitch, but it even happens when I talk to my wife! Maybe it's "telephobia".
Maybe it has to do with the receiver too close to the mouth. Have you tried using a speaker phone to see if you cough then?