No harm in revealing this if bin Laden's people are already on to the danger of making such calls, as they undoubtedly would be after mass arrests. We could see if they are on to the danger if they have stopped making such phone calls.
Only problem then is that the Islamofascists' failure to make such calls over the anthrax would then be of zero significance in supporting the conclusion that they are not involved in the anthrax, and apparently U.S. intelligence agencies are using this silence to argue that the Islamofascists are not the guilty parties in connection with the anthrax.
Well the intelligence community might have other evidence from the Al Qaeda group that is related - but not directly revealed here which would point the finger away from a direct connection in this case. I'm not saying that the government is necessarily on the right track wrt the anthrax attacks - but no one in government seems to take a direct foreign connection too seriously. Perhaps they do have intercepts from domestic groups that they aren't leaking yet which is leading them in that direction.