With old-fashioned, mechanical exchanges, some taxi firms would instruct their drivers that (when ever they were 'out in the sticks') to find a rarely-used phone box and to dial the number of a rival company, leaving the phone off the hook. That would prevent the rival company from taking bookings for several hours, or even days, until somebody replaced the receiver.
When electronic exchanges were introduced, BT introduced a system that automatically cuts off 'silent lines' after a few minutes specifically to prevent mis-use of the system. So, if you're line isn't 'self-clearing' there's a fault somewhere.
If you're with BT, phone 0800 800 151 to report the fault. Otherwise contact your service provider.
Chris