Network isn’t even the only egress route out of a cellphone. They have sophisticated radios, so a low-level (e.g. on-silicon) backdoor could send your data out

Network isn’t even the only egress route out of a cellphone. They have sophisticated radios, so a low-level (e.g. on-silicon) backdoor could send your data out to a nearby agent using all manner of electro-magnetic emissions. You just have to trust the manufacturer and its supply chain, and that applies to open source too.