Generally Whatsapp and Telegram have been reported as working. Please don't force me to do a messenger comparison here, mainly because there are good and extensive lists available on the web. Our documentation (https://docs.nitrokey.com/nitrophone/apps.html) suggests Matrix & Signal. It's also very challenging to answer you "how to communicate securely", clearly it's not as simple as ensuring both are using a NitroPhone, but this will at least set the bar higher for a potential attacker. For Telegram, please keep in mind that (as far as I know) they keep all (group?)messages on their servers, which is not necessarily what someone would consider "secure".