why does the registration option does not show up on a BT 3500 handset?

If you purchased a multi pack then each additonal handset has already been registered with the base station which is why the one you have doesn't show registration. If you only purchased the main station hand set and not a multi pack
then yes you will have to register the additional handset.

As you haven't got the user guide go to www.bt.com type in search box top right 'Graphite 3500 user guide click on graphite 3500 name in blue ..... scroll to documents select the 3500 user guide pdf down load and save it to your coputer. Look up the registration instructions on page 38.
Follow the instructions.

Bob Ex G.P.O/BT retired
September 2012