Phone, RJ11 and BT Adapters Explained

UK phone sockets take a BT plug, but most routers, and many phones and phone systems, use the smaller RJ11 plug. This guide explains which cable or adapter connects your router or phone, when you need a microfilter, and what changes when your landline moves to a digital phone service.

BT, RJ11 and the other phone plugs

Plug	What it looks like	Found on
BT	Flat and wide, with the clip on one side	UK phone wall sockets and the leads of most UK phones
RJ11	Small clear plug with the clip on top, 6 positions	The broadband port on routers, phones, faxes, microfilters, filtered wall sockets
RJ45	Like RJ11 but wider, 8 contacts	Network cables, office phone systems and patch panels
RJ10	Smaller still, 4 contacts	The coiled cord between a phone and its handset, such as our 2m handset cord

Connecting a broadband router to the wall

Your wall socket	What you need
Master socket with a separate small broadband (RJ11) socket	The filter is built in: an RJ11 to RJ11 modem cable from that socket to the router. No microfilters needed anywhere
Plain BT socket	A microfilter in the socket, then an RJ11 cable to the router; the phone plugs into the filter's BT socket
RJ45 socket (office or patch panel)	An RJ11 to RJ45 cable
Full fibre (FTTP)	No phone cable: the router connects to the fibre box with an Ethernet cable. See which Ethernet cable

Microfilters stop phones crackling and broadband dropping on the same line. On a broadband line without a filtered master socket, fit one on every socket where a phone, fax, alarm or TV box is plugged in. We have plug in, leaded and tail types, and an ADSL surge protector.
Moving the router for better Wi-Fi: use a longer RJ11 modem cable, up to 30m, or a BT extension lead. For the best broadband speed, keep the router connected to the master socket and the run as short as you can.
A router with an RJ11 port and a plain BT socket, with no phones on the line or the phones filtered elsewhere: a BT to RJ11 cable plugs straight in.

Digital phone lines: plugging your phone into the router

The UK's old analogue phone network is being switched off, and landlines are moving to digital phone services that run over broadband. Your phone then plugs into a phone port on the router instead of the wall socket.

If the router's phone port is RJ11 shaped and your phone has a BT plug, use an RJ11 plug to BT socket adapter. It is wired so the line reaches the centre pins of the RJ11, where the router expects it.
To use more than one phone on the port, or a phone and an answering machine, add a BT double adapter.
Microfilters are not needed once the line is digital or full fibre.
If the phone is in another room, a BT extension lead from the adapter reaches it.

Some alarms, care alarms and card machines rely on the old phone line, so check with your provider before the switch.

Crossover or straight BT to RJ11 cables?

A BT plug carries the phone line on pins 2 and 5. Most routers, phones and modems with an RJ11 socket expect it on the two centre pins. A crossover cable connects those, so it is the usual choice, such as our 2m or 10m crossover cables.

A straight cable, such as our 2m straight cable, is wired pin for pin, for equipment that specifically needs it. If you are not sure, choose crossover. If a cable gives no dial tone or no broadband, the other wiring is the first thing to try. The same choice applies to BT plug to RJ11 socket adapters, which come in straight and crossover versions.

Splitters, extensions and office adapters

Double, triple and 4 way adapters turn one BT socket into several, such as our double, triple and 4 way adapters. Everything plugged in shares the same line and number.
Extension leads (BT plug to BT socket) move a phone or router away from the socket, from 2m to 20m.
Office RJ45 outlets: a full master PSTN adapter turns an RJ45 data outlet carrying a phone line into a BT socket, and a PABX adapter connects BT plug phones to an RJ45 phone system outlet.
Making your own cables: RJ11 6P4C and RJ12 6P6C crimp plugs.