DIY basic mobile phone

I am a 35 year old academic, wanting to explore the world of tech some
more. So the task I have set myself is to learn how to build and
program a mobile phone (only for calling and SMS over a GSM network).
I am in uncharted territory, so I need your help please.
Ideally want to run the phone over a LTE network.
I had a look at some part from your website, but need some guidance
please on what I need.
I observed this product, would this be suitable?
I want to go down the road of Raspberry Pi, but I am open to Arduino systems as well potentially.

Best regards,

You haven’t said what product you looked at, or provided a link to it.

Basically, most of the communications board can interface with any of the popular SBC/microcontrollers using one or more of serial, SPI, or I2C protocols. What determines the best choice is what you need to do, and how much processing is required to do that.

Also, remember that 2G is no longer available in Australia, and 3G is already being phased out by some providers, so minimum must be 4G/LTE.

Perhaps something like this board: