Hmm... interesting. Spain should be covered by GDPR so you should be able to say "I reject right to process my personal data" and it should stop...
I had issue with Vodafone here - they were pestering me with calls/messages... even after I switched to Digi they were calling me for a week to try to convince me to stay (it just confirmed my decission to switch ;) )
Luckily at the moment, there's still a delay after you answer the call as (I assume) you're being connected to a human. How long will this last....?
Currently, when I don't hear a voice within 1s or so, I hang up. A legitimate caller will (hopefully) call back pretty quick.