A microfilter is a device that allows broadband to work at the same time as your phone service. Without microfilters you may get problems with your BT broadband or experience noise on your phone line.
If your master socket has one socket:
You must use a microfilter for every phone socket in your home with any phone or broadband equipment plugged in (including your Hub, phones, answer machines, digital TV boxes, and alarm systems).
If you need to order more microfilters from BT, go to www.bt.com/shop >If your master socket has two sockets, like this:
You must always plug your Hub into the broadband connection on your master socket. If you plug it into a phone socket or extension, it just won’t work.