In the UK landlines almost always start with 01 or 02 so it's easy to identify who is using a landline. You can also go through the phone book (which only lists

In the UK landlines almost always start with 01 or 02 so it's easy to identify who is using a landline. You can also go through the phone book (which only lists landlines) looking for "elderly" names. People who don't bother / know how to opt out of the phone book are probably easier targets as well.