A simple method guaranteed to work regardless of how messed up the device is: Faraday cage. As long as it is sealed and entrance/exit portals closed it will prevent this. Use an airlock type of entrance to ensure it stays closed all the time. Put beacons inside and detectors for those outside to keep track of the cage's integrity. The only problems are that it can be a pain to retrofit into an existing work area and it costs money -- but it would probably be less than potential government fines for false alarms.
Wallboard in MRI rooms are lined with copper to keep external RF signals out of the scan room. Without getting into the weeds of how MRI works, a clean RF signal is needed for imaging.
It would seem that a copper wallboard lined room at the repair facility would be able to keep the signal from getting to the cell tower.
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It would seem that a copper wallboard lined room at the repair facility would be able to keep the signal from getting to the cell tower.