A bell does not need to be filtered.. a filter only spilts the analogue speech signal from the BB digital signal so the white noise of the modem/router is not heard on the phone , which is why you need a filter on each phone , to call it a filter is a bit of a misnomer as it is really a splitter.

Could it cause an issue though, by proxy?

The trouble of this problem is that it is intermittent and by that I mean every 1-2 months for 2-3 days. Not a major butt ache, but when it comes its frustrating.

Equally by the time someone from BT turns up it'll likely have corrected itself.

In reply to a post by MHC:

Back in the GPO days ... you would be charged line rental including the first phone and if you wanted a "non-standard", different coloured phone, or the "Trim Phone" or an extension, or external bell, or ... there would be an additional monthly charge!

Registered hard of hearing used to get a bell FOC.

Doubt it is causing noise though, unless the noise is a constant hum caused from unbalance.

If an eng. turns up and everything tests ok then your CP will be charged and they may pass the charge onto you.