The idea that it is legal on your on property is from right of use laws and common law pertaining to property rights. Before aircraft, spacecraft, and radio's you had rights to the air above you. But the FAA and FCC have more or less taken much of that right away. Courts have thrown out cases where the signal is contained within the owners property, but there are exceptions and it is not from written law but common law, which is always at the mercy of the court. So you are at risk (though small) even if you hava a Faraday cage.
Even then you might be subject to civil court if someone died or injured because they could not call 911 from your property. Especially if there were no warning that cell phone transmissions were blocked.