thmf
sry i was about to write something longer, but honestly i dont know exactly,
and i think it really depends on the use case
like do they need to be anonym, are they a public figure, are they like idk in a illegal organisation...
like hisbollah used pagers and walkie talkies for a reason (i think it was to not risk getting their location exposed tho, not hacked)
but yes probably they should not use a phone at all together with any critical information, if they dont have to for some reason
( i could imagine they could try to run some kind of own minimal system to access the internet or to send other kind of messages, i know it is definitly possible but not sure if it would be more secure (since the hardware definitly would not), but the longer you use it, the more likely is someone finding exploits etc. keeping your exact system secret is then a challenge aswell)