You can (and some do) insist, that all phones and other digital devices be left outside or out in a well isolated place like a fridge. Furthermore you can use methods of encryption for both physical notes and digital notes. Neither is completely safe from being compromised, but then again hardly anything truly is.

If you want to be fairly certain to not be monitored, always leave digital devices behind and have meetings in varying locations with few or ideally no other people around. Only discuss confidential information with people you know and trust. Even then, experience from places like East Germany proves, nothing is truly safe from being compromised if enough time and energy is spent on the issue.