And cellphone signal blocking is localized, short range, same as WiFi.
Put up a sign to that effect in your business premise.



An idea just occurred to me.

You notice those electronic anti-shoplifting interrogators at the
checkout counters?

Why not include a circuit in every cellphone that will cause the
answer bell to ring when anyone carrying a cellphone crosses one of
these interrogators. Maybe have the circuitry activate a different
ring tone or ring pattern from the normal call ring. Its extremely
short range and won't interefere with normal cellphone use.

It will set off a cellphone bomb or at least ID a suspicious cellphone
owner who can be asked to show the phone (any cellphone modifications
will be noticed immediately) or can be called aside for further
inspection. This will enable security people in places like train
stations, bus stations and airports to quickly screen crowds.

The same interrogating circuitry could be installed in metal detector
security gates.

Its rare to be able to go anywhere without encountering one of these
interrogators these days. A bomb carrying cellphone triggered
terrorist will have a hard time moving around without attracting
attention somewhere.

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On a different discussion point, picture the recent Spanish train
bombings (10 set off.) Had the train installed cellphone signal
blocking equipment most of those bombs would probably not have been
set off. Calls on emergency workers commuting by train could easily
have been relayed by the train's onboard PA system. Calls out can be
made from on-board public pay phones. Or can the signal blocking be
effective only for incoming calls and leave out going calls
unrestricted? Airlines blocks incoming calls. Outgoing calls can be
made from anywhere during flight through the aircraft's phone system.