If you're in the UK the phone handsets will not be fitted with RJ-11's, they will have the UK standard 431a plugs (usually) which do need an adaptor.
Three adaptors are available
(1) for use with PSTN lines that are patched across the Cat5 scheme. These contain the neccessary capacitor for the ringing and a resistor for BT line testing.
(2) for analogue telephones. These contain a bell capacitor but no resistor.
(3) for digital telephones (most dedicated systemphones). These dont contain any extra componants.
All three adaptors are readily available from lots of suppliers - Maplin, Nimans even BT, wherever you buy your cat5 from.
NB: If you have a lot of phones to connect, it may be cheaper to replace the phone linecord with an RJ-45 option, either buy the leads ready made or invest in a bag of rj45 plugs and a crimper!