I am with Jaime on this one, rather than changing the number that they dial from the pay phone (for instance 1200), translate that same 1200 number into the number of the PD Office number.
Apply this only for the payphones by means of a translation pattern that is ONLY available to the payphones (by means of changing the CSS on the payphone line).
This means that ANYTHING else calling 1200, will still get through to the switchboard as per usual.
cheers
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