It would cost you NOTHING to simply reject callers who present a number that you don't know.
Most cell phone providers offer FREE call blocking services, call yours and inquire.
As suggested by others, simply don't answer or reject numbers you don't know.
There are many FREE solutions, such as turn the phone OFF when you aren't calling.
Turn the phone on when you wish to call someone.
That said, if you're getting something for which YOU aren't paying, you have to abide by the rules or limitations that your benefactor sets; in this case the US Gubmint and the taxpayers who fund your FREE for you device.