There are exceptions when silence is acceptance. These are cases in which silence is a manifestation a willingness to enter the bargain.
As a preliminary consider a non-contractual example. Suppose Edwards has applied to Mason for a job; Edwards lists Dixon as a reference.
Mason calls Dixon and says, "Tell me the three most positive things about Edwards." Dixon responds with complete silence. Mason infers--as Dixon intends--that Dixon has nothing positive to say. Dixon's silence is meaningful.
There similar examples in the offer and acceptance context.