Welcome to the forum.
Yes, you are correct that by having the On-Call feature, you will get an additional Sim Slot for your in-car Data requirements as used by:
1. Car's Sensus system
2. Data for the in-car wifi hotspot (that turns off when the car's systems power down)
3. Traffic Updates for the car's Sat Nav
I have just purchased for my new XC60 a Blackvue 750S 2ch as an upgrade from my previous BlackVue 650 that worked without fail for last 3 years.
My configuration is to use the Blackvue Power Magic Pro Box as the interface between the car's battery & the camera (my dealer wired this for me) c£40 that I have set for 6 hrs recording limit with the camera's "cloud" feature turned on and my home wifi set as the "Garage" setting which is also the priority mode with the in-cars wifi hotspot set as the lower priority.
https://www.blackvue.uk/collections/...ower-magic-pro
The above means the cloud feature of the Blackvue camera is only available:
1. When car's systems are powered and camera is connected to the wifi hotspot.
2. When car is at home with the camera still powered for the 1st 6 hrs via the car's battery using the Power Magic Pro Box.
So if you are looking for a more permanent cloud availability option, you could replace the Pro magic cheap box with the below Blackvue battery option that will not just only power your camera but also a wifi dongle (with built in sim) via one of the USB sockets...............but then you will end up with two sims, etc, etc.
https://www.blackvue.uk/collections/...ry-pack-bv-112
Re the question of what sim card to use, then suggest the below option as used by a lot of Volvo forum members.
https://www.amazon.co.uk/Three-Mobil...01M3VJ2B2?th=1
As regards future forum posting, I suggest you use the main "General" section that is where 95% of the members visit for our models as below.
https://www.volvoforums.org.uk/forumdisplay.php?f=201
Hope this helps.