nh3 and ph3 are from gROUP 15 AS THEY WILL BE BASIC DUE TO DONATION OF THE LAONE PAIRS THEY ARE BASIC LEWIS BASE YOU ASKED WHY BECAUSE THE LONE PAIR IS IN S ORBITAL  WHICH IS ACIDIC .YOU HAVE TO USE THIIS AS COMPARISOM WHEN THE LONE PAIR IS IN DIFFERENT ORBITALS LIKE S  P D F ETC HERE  THE LONE PAIR IS IN S ORBITAL  SO YOU HAVE TO SEE THIS. IT HAS BEEN A PROBLEM THAT WHEN WE SEE A SUBSTSNCE ACIDIC WE FEEL IT IS ALWAYS ACIDIC BUT POINT TO NOTE IS ACID AND BASE ARE ALWAYS COMPARABLE . ONE CAN BE ACIDIC TO ONE AND AT THE SAME TIME BASIC TO THE OTHER.    NOW TAKING NH3 AND PH3  THE BASIC CHARACTER IS DUE TO THEIR SHAPE AND DISTORTION ACTUALLY THE BAS WHEN DONATES THE LP THEY ACT AS LIGANDS FOR COMPLEXES AND DUE TO VARIOUS DEGREES OF DISTORTION DUE TO VESPER MODEL THEYE DONATE TO VARIOUS DEGREES      YOU CAN SEE IT JD LEE INORGANIC PAGE NO 489 
						

							
NH3 is more basic than PH3 as N is more electronegative than P.  (Electronegativity decreases down the group)