A GHz-level spectrum analyzer placed near a cell phone would pick up the ELF modulation, if programmed to look that low. Most systems filter out anything below 60 Hz to reduce noise.
Properly set up, it would be easy to see.
Basically, what you are hypothecating is that there is a general rise and fall of the entire carrier and signal of 4 to 14 Hz. Note: "One-Miss-is-sip-pi" is 5 Hz, for reference.
Note: there are radios that "listen to the earth" that operate way, way down in the ELF band. They cost about $200 and are used primarily by people into crystals and other Gaia-based faiths.
If a wealthy anon bought one, this would be an easy test.