One of my prior staff/friends (ex Gov spook hacker) launched a service in Canada a few years ago, one of the biggest drawbacks i saw, and it turned out to be true, is that the phones continuously need updated software so even if they are secure today, they may not be tomorrow, then add in cash flow issues and you have a phone that becomes possibly insecure over time, and/or a brick.
There is ofcourse a hardware direct variant, but there's easier ways to communicate today without using speech.
For example something i sent over to the FEDs (various countries) was the concern i had with spun up blockchains with encrypted messages in the DATA side, etc.
If you are︀ operating in the legal side but just want privacy, go with something standard off the︁ shelf where sold as x.
If working in the Grey/Black you have the issue highlighted︂ above, but also possibility that its a rat trap (HA HA).
IF I WERE A︃ CRIM and/or state actor, i'd bury communications in public messages/boards/transactions or go deep and generate︄ my own. - I don't advise that - but coming from the field (cybersec,hacking,ai-data-harvest etc)︅ can say from exp and logic none of the rest would be 'secure'.
Should add︆ to that - my concern was that terrorists could utilise the technology - they can︇ - easily - cheaply - and quickly move on to other instances - whilst storing︈ in countries with strong privacy rights - or state actors home countries - but accessible︉ everywhere - akin to the dark web but visible on the open web.
Hiding in︊ plain-sight.
Technically a product/service for a security/privacy company i guess also.