That isn't sufficient. I have never done that, yet I still receive the texts (oddly, they usually address my sister, and sometimes my mother). It's my understanding that voter registration lists in the US are public.
Your family probably fell for some sort of scam where their contacts were skimmed.
Voter registration lists are public, your phone number is not.
I get texts from Trumpsters because I worked at a place with a public sector union and some ass gave the GAL to a anti-union group, which subsequently sold it to some other groups.
