first pair for connecting the terminal equipment to the PSTN and the second pair for the connection of a line towards the next socket. However, the presence function found in Swedish Standard sockets is lost, so disconnecting a telephone from its socket requires replacing it with a dummy plug so the sockets further down in the chain will remain connected. 6P6C modular plugs are also used for telephony, but only for certain telephone exchange systems - never for direct connection to the PSTN. ISDN telephones and outlets always have 8P8C modular connectors. See also Telephone plugs Registered jack Reference and further reading Description of Network Interfaces ; Analogue Access to PSTN from Telia (pdf) Telephone connectors