See my other comment for more details: https://news.ycombinator.com/item?id=41704119

To look up the origin use a website like https://www.freecarrierlookup.com/

Then you can go to that platform’s page for reporting abuse or spam (find via search) and fill out their form. Sometimes those platforms will say they can’t do anything since it is a different platform that isn’t a direct customer but yet another platform, so ask them to name them. You may then need to find that other platform’s reporting page.

Just be aware that after all of this, you may not actually fix your problem. Some of these companies seem to repeatedly send spam because they have customers that just perform the same abuse from a different phone number or different account with that platform. That’s why the reports to the FCC and FTC matter, to investigate platforms for broader issues.