If they wanted low key interception of civilian terrestrial radio signals, seems like it would be better to just rent a Uhaul or RV, load it up with radio gear, and drive it across the country.
As long as you are in airspace and not in space (and those balloons weren't), it's not legal to violate airspace and wiretap signals without permission. So U-Haul wouldn't be much different. And the balloon was very slow as well, at least a truck you can drive and park exactly where you need it.
Low Earth Orbit satellites of the US are 100% intercepting global cell phone data and geo locating their source. Plenty of companies are even building direct cell phone service from satellite to the ground, including SpaceX.
