The most effective security measure you can take to protect a computer is to simply unplug it from the Internet. It's called an "air-gap," and it's used on computers with very sensitive information to make them unhackable. Except they're not.
In most sensitive cybersecurity environments, it's already against the rules to bring in smartphones. Intel's manufacturing employees, for example, cannot use phones with features other than voice and text messaging. Last year, academics in Israel managed to hack data from air-gapped computers by using smartphones' FM receivers to intercept radio signals coming from the computers' video cards. Hence the ban on smartphones.
Now the same researchers have proved it's possible to hack a computer that's not connected to the Internet with a simple voice-and-text cellphone. But it requires a different method.
Malware must be installed in both the cellphone and the computer, but once that's done, the basic cellphones can intercept and decode electromagnetic radiation coming from the computer.
Only a small amount of data can be transferred using this method, but it's enough to hack passwords or encryption keys.