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?
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".
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.