A microfilter is used on broadband connections to separate the voice and data frequencies that arrive down your telephone line. If you don't do this you will find that your broadband frequently disconnects and you get poor service.
Therefore it is really important that you use microfilters in your property. You will need to filter everything that you plug into your phone sockets - this includes any other telephone extension sockets that may be in your premises! So as an example, if you have a telephone socket in another room that you want to plug telephony equipment in to (for example a phone, credit card terminal or fax machine) you will first have to plug this into a microfilter and then into the telephone socket.
The golden rule is: if you have any telephony equipment plugged directly in to a socket whilst trying to use broadband, then you will encounter problems with your broadband service.
There is one exception to this, however.
If you are on Fibre To The Cabinet (FTTC) services your main socket in your premises may look like this:
There is a small socket at the top and then a standard telephone plug socket underneath. In this instance you have a microfilter already installed on the line as it enters the property, and you just need to plug your modem in to the top and any telephone equipment into the socket below.
As the signal is filtered as it comes in to your property you can go ahead and plug telephone equipment in to any extension sockets without a microfilter.