User level migration of telecommunications to the internet

I have double-posted this to the Asterisk community and this blog.

Suggestions for implementation patterns and tools are welcomed...

Please do not suggest an online service. I am looking for a bullet-proof DIY solution of high quality and at a low cost.

I want to set up a physical connection between an Asterisk server to my mobile phone provider represented by a SIM card and a (national) mobile phone number (LTE or 5G) with SMS/MMS and other services. The physical gadget holding the SIM might be an Arduino with a GSM module, a raspberry pi with a broadband modem, or a mobile phone connected through a USB port. To be placed on my desktop or elsewhere 24/7. The sim card itself should be able to provide both internet, and phone calls, and SMS/MMS messaging. MMS messages capability is more a question of parsing SMS messages than coming up with a completely new read/write capability for the solution.

Aim: To be able to receive and send SMS/MMS and pass mobile phone calls seamlessly on a softphone app iOS/Android on a worn smartphone disconnected from all but a single band, i.e. 1800MHz, or connected to the internet using wifi/BlueTooth or USB tethering.

Staying disconnected I get a super safe system with the potential to add security measures like PiHole and using proxy servers for added security, getting rid of js trojans from both authorities and private crackers. It is also a system that makes it harder to locate the caller.

Daily use:

I can store SMS/MMS messages on the server at home, while I await a hot spot opportunity. Going offline several times throughout the day is not that important, I do not work as a receptionist. The Asterisk server will provide the voicemail capabilities of professional companies for me as a private user, and I see commercial potential in the solution or at least a market-changing potential migrating telephone traffic to the internet increasing degree of security through encryption techniques and advanced cisco. Puts a competitive strain on the telephone providers, eavesdroppers, and damaging unconstitutional authorities.

On another softphone app (same or different worn smartphone) I can add a business-related link to a more traditional appliance of the Asterisk server and all the capabilities needed for professional service; an application of Asterisk described elsewhere.