It sounds like you are not getting the proper disconnect supervision from your telco provider.

Here's a simple test... call into your office and after the call is answered, hang up the phone you initiated the call from. Now watch the line on the keyset. If it doesn't go out quickly... telco is not sending the proper signal to tell the phone system the caller has disconnected. Here in the US it's called CO Disconnect... but I'm not if it's the same thing on your side of the pond.

I'll see if one of the Mods in the Telephone Services section can provide some better information.