If you read the posts on the whocallsme website you will find that the calls are from telemarketing (or similar) companies.  These companies want to hide their real number as they generally do not accept incoming calls.  I guess it's the equivalent of the US '555' area code that is used in films and doesn't exist.

I get calls from 1004 (Telefonica) and recently 1444, both telemarketing calls.

I'm sure it's nothing sinister. Any proof that it is???