I don't need any filters at all as I am on cable but from what I have heard/learnt you need a filter for every telephone socket you have, regardless of whether there is a telephone plugged in or not.
Dell XPS 13 9370 (2018) review
No need to scan sketches into your computer with Moleskine's new smart pen
WWDC history: Apple's product launches since 2005
Comment importer des contacts d’un iPhone à un autre iPhone ?