I have a 1.7 Mhz cordless phone. Yes, I know it's ancient, and subject to
interference, and readily overheard, but I've always liked its sleek design
and the swappable sealed lead acid batteries that means that the handset
itself never sits in the charger. But is has just 10 channels that I have
to switch through manually.

I recently installed DSL. I installed the filters that came with the DSL
device, but sound quality on the cordless phone is much worse than before.

I was wondering if such cordless phones require a different filter.