This is from the BT website and is in according with the BT Engineer I spoke to:-

"t's vitally important to make sure there's a broadband filter plugged into every active telephone socket in your house, including sockets used by Sky boxes. Active sockets are ones with a telephone or other device such as a fax plugged in. You don't need to bother with sockets that aren't being used for anything.

You don't need microfilters if your master socket has a broadband faceplate. A broadband faceplate has two sockets - one for your phone line, and another for your broadband.

You also don't need microfilters if you've got fibre-based broadband, like BT Infinity. You'll know you've got fibre-based broadband if your broadband setup looks like this, with your BT Home Hub plugged into an Openreach modem using an Ethernet cable."

So if you are right in saying you don't need one if the master socket has a broadband faceplate, with two sockets - these are usually fitted by BT if you have their broadband - many people do not have BT and do not have this faceplate fitted and this is why internal filters are recommended by ISP's.

So if you do not have one of these fitted - use a microfilter is the advice - certainly worked for me.