WOW! It sound like everything is a problem to use - and everything on your phone then is sold or just used everywhere. Now I'm really afraid to use DUO (or anything else)
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OK.
I am going to toss my hat into the ring on this one.
First, let me answer the question for you.
The app that I would suggest to use is called Telegram.
Don't use the Google PlayStore version, as there is a FOSS version available here:
Telegram (Telegram is a messaging app with a focus on speed and security.) -
https://f-droid.org/app/org.telegram.messenger
Your person with the (cr)apple (sorry, I couldn't resist) can get whatever version is available for that type device. (I would post a link, but I have no idea about iphone- other than the fact that I don't (won't) have one.)
Telegram will let you call, text, share, and store unlimited media for free- and without ads.
I use it worldwide (USA, Europe, India, China) every day, and it is great.
You must go through all the settings to secure and personalize it as you see fit.
It will, at first, ask for your contacts. This is so that it can tell you which of your contacts are already on Telegram.
This can be changed in the settings.
Some countries have banned Telegram, because they have refused to give up the encryption keys.
Also, there is a full encryption mode- called 'secret chats'- where the encryption keys are only device to device.
Those chats only appear on the devices involved in the chat, and can only be decrypted by those devices.
Just so you know, your Contacts app has most likely already uploaded all of your contacts to Google already- so your worries about Duo having them are kind of moot.
Not that I am in favor of Google having any information at all.
But a Google app is a Google app.
If you give one Google app a permission, you might as well give them all that permission.
It is basically understood that if Google can't get your information via one route they will get it through another.
So try Telegram.
It is an easy set-up, easy to use app.
I have used it for a couple of years now, and I wish I could get all my friends on it instead of the social media junk that they are on.
Let me know if you try it and have any issues or questions about it.