Make call remotely from one phone through bluetooth using other phone?
I'm also trying to achieve a similar task with two handsets.
I use unlimited data as my primary home internet. (trying to save money going without broadband) but to do this I have to USB tether my phone (Pixel 3) to my home network.

I want to instead use my old Android handset (Galaxy s3 mini) USB tethered to my home network and bluetooth remote SIM access from my Pixel 3 to act as a virtual 3G modem.

Currently I have to either swap out the SIM card or just leave my Pixel plugged in by a window with good signal.