Cellular iPad - Cellular phone number in iMessage account

Hi guys

I'm probably going to buy a new iPad Pro with cellular support, but first I need a clarification regarding iMessage/Facetime on my account and hence the provider I've to choose.

My main question is, if the phone number of the iPad cellular sim gets automatically added to the iMessage/Facetime services of my account as below described:

The carrier of my family allows "multiple device support" but only if you have a specific data plan (for the regular mobile services) and these data services are pretty pricey (starting at 70$ a month) and I do not have such a data plan for my iPhone, hence I cannot buy these services. However my parents do have such a data plan subscription and the option for multiple device support is only 10$ a month for them. My idea is to add this "multiple device support" subscription to one of my parents account but using the sim card in my own iPad and pay them the monthly fee and this probably will work but I don't know if the phone number of the cellular number (which is obviously the same as the number as their regular mobile number) gets automatically added to my iMessage account (they both also use an iPhone with iMessage activated).

It's kind of important for me because I don't want to receive iMessages which are supposed to be delivered to my parents iMessage account.

If this would be the case, then I have to switch to another carrier (at least for the iPad data plan)

Thanks for any help.