A lip slur on a trumpet? If you mean to fill the fill the total space from one note to another, it's seldom precise. If you're correcting intonation, it often is a necessity. In full orchestration with a duo of trumpeters overlapping each other and a touch out of synch the lip slur from one note to another sounds great. Ah, the beauty of a trombone slide for slurs from one note to another.