Any broadband chip since 3G ships with proprietary drivers which have backdoors. I tried to build an open phone, worked for one of the major telcos, and could never get around the driver issue in trying to make an open phone.

BUT sophisticated attackers like US or Israeli governments (and I assume Russian or Chinese but I don’t have direct experience with these) don’t need these backdoors, getting anywhere near your phone is enough to root it to allow installation of spyware, according to my CSO who worked in naval intelligence. There are simply too many vulnerabilities for there to be a hardened device in the consumer space. Some are better than others (Apple) but as Bruce Schneier says, if you are worried about this sort of thing you really have to be totally disconnected from the internet and exchange encrypted physical media.