I have two smartphones at the moment: iPhone SE and a Sony Xperia XZ1 Compact. But I loathe phones in general and I want to get rid of them as soon as possible. I don't use my phone very often, and if I can then I do anything on my computer.

The only thing that prevents me from getting rid of my (primary) phone is actually WhatsApp messaging. Almost all of the contacts I work with use WhatsApp and it's the primary way I communicate with almost anyone I know.

I use WhatsApp Web on the Thinkpad, however WhatsApp requires my phone to be charged and connected all the time.

I just subscribed two new phone numbers for myself, and one of them is for my Thinkpad, but I been told by the provider that it's only for data (LTE) i.e. internet connection, and not for actual cellular calls.

My Thinkpad has a SIM card slot that supports 4G and LTE with WANN. This is from the Lenovo specification of my X270:
Which is what my exact model is equipped with. Can this potentially replace my phone in the sense that WhatsApp will accept my Thinkpad's mobile number and stop asking my iPhone for connectivity?

I have Linux Debian (testing) running on my machine, I have plenty of space so I can potentially install another mobile OS on a different partition or on an SDXC card with 128GB and occupy the SD slot with it all the time.

I'd prefer to run a virtual machine with a very basic mobile OS just enough to launch WhatsApp and then move back to my Debian desktop and do the texting/messaging form the Web client. Is any of this workable?

You could get a virtual phone number from Google Voice and hand that out for voice calls and SMS. it has an option to answer calls from the browser when logged into Chrome, or they just leave a voice mail. You can read and reply to texts from the web page.
Not sure about WhatsApp as I don't use it. Not sure if it will take the GV number.