I can only assume the parent poster is referring to the so-called baseband, that is running on the modem processor and indeed is underneath all other OS. To know exactly what each baseband does or doesn't do, just ask people who will tell you!

My guess is he's talking about the management engine.

Which, if one's comfortable with the HAP bit flip as a solution to that problem, then one should arguably also be comfortable with bit flips controlling baseband radios which still haven't been demonstrated to transmit when disabled.

So it doesn't provably really fit description "phones home anyway".