Unfortunately, there’s not much you can do to improve the security of your cellphone. Unlike computer networks, for which you can buy antivirus software, network firewalls, and the like, your phone is largely controlled by others. You’re at the mercy of the company that makes your phone, the company that provides your cellular service, and the communications protocols developed when none of this was a problem. If one of those companies doesn’t want to bother with security, you’re vulnerable.