I believe the rules allow the serving team to call a let when the serve clips the net (usually called by the server's partner at net). But how often does this happen? I hardly ever call a let serve when my partner serves because I figure the receiver's partner can see and call the let serve. But sometimes the serve will barely skim the net and nobody hears it except me (since I am the player closest to the net). At that point I have to decide whether I am 100% sure. If not, I say nothing.