The number you need altogether depends on whether you have phone extensions and how they are connected.

The actual rule is that the signal from a phone should pass through one and only one filter from the phone to your exterior phone line and similarly the ADSL device should pass through one and only one filter from the ADSL device to the exterior phone line. The ADSL signal cannot pass through the phone portion of a filter.

In this context a phone connection includes a phone, a fax, a dial-up modem or a connection from your SKY satellite box to a phone line. You may see these called POTS devices; which stands for Plain Old Telephone Services .

You can have more than one POTS device connected to a phone line, but you can have only one ADSL device connected to a phone line.

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First let's identify some plugs as you will need to recognise them.

This is a BT phone plug. Devised by BT when they thought they could still hold a monopoly on phone connections so it is different from the RJ11 phone plug used in the USA and many other places in the world. It has 4 or 6 wires. It is designed to 'snatch' out of the socket if you trip over a phone wire.


This is an RJ11 phone plug, as used for phones in the USA and many other places in the world. It has 4 wires. Since most broadband ADSL devices are designed to be used in the USA they usually {if not always} have RJ11 plugs and sockets to connect the broadband. In some ways this is convenient as in the UK you cant plug a phone or ADSL device into the wrong side of a filter. You may remember that when you used dial-up Internet, you had one of these plugs on the modem end of your connection wire.

This is an RJ45 plug. It is similar in design to an RJ11 plug but is wider and has 8 wires. It is mainly used for Local Area Network [LAN] connections (called CAT5 or CAT5e), to connect computers to hubs or routers. However office buildings nowadays are often wired with what is called 'structured wiring', whereby all the signal wires are in place, using RJ45 outlet sockets and can be used for phones or computer networks. All the wires go to centralised patch panels from where they can be plugged into the relevant facilities. Phones with BT plugs need a little adaptor unit to allow them to plug into RJ45 structured wiring sockets

This is another RJ45 Plug, which has a shroud moulded onto it for extra strength.

An RJ11 plug can plug into the centre of the wider RJ45 socket, using the centre 4 wires. It is a bit wobbly sideways but normally connects OK.

Revised on 15 May 2006

The Easy Solution

If you have no wiring skills then you can use plug in filters.

They look like these:















The input side has a BT plug to connect to a phone socket. There are two outputs first a BT socket for phones and second an RJ11 socket for broadband ADSL devices.

(a) if you only have one phone (or ONE master unit for one or more cordless phones) and and the ADSL device is near the master BT phone socket then you only need one filter.

If you have phone extension(s) you basically need one filter for each phone or fax etc, plugged into the extension socket.

{well strictly speaking if an extension socket has ONLY an ADSL device plugged into it you can use a UK to USA (RJ11) phone adaptor (or an old dial-up modem lead) there instead, because the filtering action actually only affects the PHONE output of the filter. The ADSL (RJ11) output of many such filters is simply direct wired to the input wires. However all-in-all it is probably best to use a filter here too, just in case, sometime in the far future someone in the household plugs a phone as well into that socket using a BT socket doubler}.


Click to magnify

If possible it is best to connect the ADSL device to the master socket as above, but it is usually possible to use any of the extensions for the ADSL device. If you are having the extension points installed it is recommended that you ask the wiring technician to use twisted-pair wiring rather than the more common flat pair wire. Flat wiring picks up interference and may prevent the broadband working or may reduce the maximum speed available on the broadband.

Putting the ADSL device on an extension is like this:


Click to magnify

If this does not work out well and your ADSL device cannot be near the master socket, you may need to rewire your extensions using twisted pair wiring or install a twisted pair extension for the ADSL device like this:


Click to magnify

You can have two phones plugged into ONE filter if you use a phone doubler plugged into the filter. Do not do it the other way around i.e do not plug a filter into a doubler as one of the sockets will then not be filtered.


Click to magnify

Revised on 21 Sep 2006