If you have your phone line enabled with ADSL (Broadband) then every extension must have a microfilter.
The quality of these microfilters varies considerably, buying a standard one from your local store for most people is fine, however many lower quality versions have an impact on sound quality which tends to be more noticeable the longer the cable run is back to the phone exchange.
In the UK the ADSL Nation XF-1e is developing a reputation as probably the best microfilter you can buy. It's designed to work with just about any piece of telephone equipment you can find - including those that cause problems with other microfilters. This microfilter exceeds the specifications of BT SIN346.
The Excelsus Z-420P2J which sells for under £5 in the UK is also considered very good.
Actually, the best quality microfilters are not the most expensive, the two I have highlighted above can be purchased in the UK at a retail price of under £7
When it comes to B&O equipment such as Beocom & Beotalk products, the only product to date where there does seem to be a problem is the Beotalk 1200 which can provide an intermittent problem where someone leaves a message, but the News indicator does not register, so you do not know that a message has been left. This is cause by a small drop in line signal which is often linked to wet weather which changes the impedance of the phone line. If you have this problem, then change your microfilter before you go any further. I know of at least two B&O dealers who have had this problem on their own store Beocom 1200's