You would have to consult the rules. One would think in a casual match, fine. In a the official rules apply match, no as generally, all calls are made by the player on the active side of the net, lines, serves in or out, etc.
However, according to the USTA site, the answer is yes, a server can call an unheard by others let ( presumably as long as he does so immediately not after a ten ball rally. )
Seems like a weird rule to me. If the receiver, doesn't hear/notice a let, it must not have been much of a let and I would say play on, but evidently, the rules say otherwise.
From the USTA site:
If my opponent serves and I call a let, but he does not agree, who has the final say?
Few people know this, but according to "The Code, a Players' Guide to Unofficiated Matches", any player (in singles or doubles) can call a service let (ball touching the net). It must be done promptly. Usually the receiver(s) make the call and many players on the serving side decide not to get involved with the call thinking maybe they were mistaken.
If the server hears a clear let and the receiver does not call it, it is best for the server to call it ASAP. Otherwise, the server hears a let and the receiver hits a winner, and after some hesitation, the server asks "Did you hear a let?" That is always difficult situation to resolve after the point has been played.
Last edited by Fearsome Forehand; 01-13-2013 at 11:14 PM.