How do I block unknown numbers on my landline?

Call your telephone service provider. Ask a customer service representative whether call blocking is an available feature.
If call blocking is available, request that it be activated. There may be a monthly fee for using the call blocking feature.
Most telephone service providers offer a variety of call blocking services, though blocking specific numbers may not be an option.

There is a service called Lista Robinson which is a spam blocking telephone and email list. I don't suppose the administrators will allow the link but here it is https://www.listarobinson.es/register-consumer.

Thanks for your replies. I am trying to get my service provider to block Movistar 1004 the annoying high speed recorded garbage. I have also asked them to block no ID calls but they are blanking me on that request.

There is a device on ebay that states it can block no ID calls, it’s from China of course. I suppose that’s a case of pay your money and take your chance.

Bronchris I tried your link and got a 404, however I found list robinson by searching. It seems what they do now is a bit back to front; you subscribe to be on their lists to be called. Unless I have misunderstood.

Legal.  I am confused. A 1004 call appears to be an out going call ( maybe a free call)

Hi: Quite right about the link. Try https://www.listarobinson.es/ instead.

What you do is, you register with them. They send you an email and then you tell them what numbers, fixed and mobile, you want blocking, which, if any email addresses shouldn't get advertising email etc. In turn they send your request to the main offenders, the Movistars, Jazztels, the insurance people, the credit card companies and a long etcetera and the idea is that those companies, which have signed up to this code of conduct, put your numbers and addresses into their not to be contacted list, they remove you from their databases and so, after a while, the nuisance calls stop.

Hi bronchris I got there in the end with listarobinson, you must enter a second surname even if you don't have one. I put in ''NONE'' and bingo all signed up. Thanks, maybe no more scammers.