If the defence does not know how the evidence was obtained, they cannot properly probe its authenticity and accuracy. If, for instance, the technique used can be shown to sometimes err in identifying the particular telephone from which a given message originates, that raises reasonable doubts about whether the right person is accused of sending that message. A simple assertion by police that they used undisclosed techniques to intercept messages originating from a particular phone, or passing between a particular pair of phones,  without disclosure of the technique used, does not allow the defence to make full answer and defence and so is unacceptable.