You have absolutely no control over what runs on your phone. You cannot even audit the software without breaking into your own device. The broadband firmware is known to be bad quality security-wise and it's tightly connected to the rest of the system. It's a walled garden where specific corporation has complete control over your privacy and you have no other choice than to trust them. Plus Apple has been already seen granting exceptional permissions to some apps like Uber for unclear reasons, so you have to trust third parties as well.

As someone who uses free software and worked closely with some mobile privacy-related projects, it's headscrambling to see iPhone users talking about privacy. I won't use it; you're free to do so, but when it comes to your privacy, it's just a delusion. At least be aware of what you've got - no security and privacy is way better than a false sense of it.