Most seniors I know have a mobile phone. How else would they be able to show off pictures of their grandkids? Also, that's how hearing aids work these days.
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.
If so, it sounds like a long-shot tactic, especially because everyone uses cell phones now.