When you want to plug two products into a single socket a way of doing this is by using a double adapter. The BT Double adapter does just this as it plugs into the existing BT socket and then provides two more sockets for telephones to be plugged into.
4 Way BT Double Adapter
If you have a BT socket that is being used for analogue telephones then all you need is the 4 way version. All this means is that the double adapter is making a connection to just four pins of the BT socket just like a standard analogue telephone does.
6 Way BT Double Adapter
If the BT socket is on a telephone system then you might need to use the 6 way version. This is because some system telephones need to connect to all 6 pins in a BT socket. As all 6 pins are making a connection analogue telephones will work on these as well.
In simple terms if you know your phone is analogue get the 4 way version. If you are not sure if it is a system phone or analogue then just get the 6 way as it will work with both.