Phone the operator. They can forward you to the operator local to that phone who is able to tell you if the receiver hasn't been replaced properly, by listening into the line. Lines that are off hook for more than a certain period can sometimes return a 'number unobtainable' tone when dialled.
BT used to use a 'howler', but this is belived to have been discontinued several years ago because of the risk of ear damage if someone picked up an extension phone once the "graduated howler" had reached full volume. [My TXE4 exchange "parked the call" after 20 seconds and went quiet after 1 minute.]
BT can request that you do not leave your telephone off the hook. If you continue to leave the phone in this manner then apparently you are blocking a line at the exchange.
BT have withdrawn their service from several people who, after being warned, have continued to leave their telephones off the hook.