Realistically, snoop on every byte passed between your phone's processor and the modem's chipset. Wort-worst case, injecting code into the phone's CPU through a

Realistically, snoop on every byte passed between your phone's processor and the modem's chipset. Wort-worst case, injecting code into the phone's CPU through another backdoor or some other exploit. Basically, taking complete control of the modem and potentially using that as a launch point for a more detailed attack. Besides that, in the US at least, your phone's cell modem does have a backdoor. Every single one of them, no exceptions. Your telco can remotely update your modem's firmware and there is absolutely nothing you can do to stop it. Telcos are allowed/required to have total control of the firmware of any modem attached to their network. Source code is never, ever available. All cell modems are a total black box. This also applies to terrestrial internet connections. Your cable modem or DSL has a firmware blob provided by your telco, no matter who you bought the modem from.