Line not hanging up "Ring/Off-hook in strange state 6"

FreePBX 10.13.66-8 + Asterisk 13.7.2


I’ve got a system connected to analog lines (UK BT lines) using an FXO card. It doesn’t seem to be recognising when the caller hangs up. If a call is placed to the system and the phones ring, then the caller hangs up, the phones will continue to ring and the external line will remain in use until an extension is picked up and put back down, then Asterisk hangs up the line. The same goes for if a caller hangs up when an IVR message is playing. The line will remain in use until the IVR timeout expires. Upon viewing the Asterisk CLI, I see the following when the caller hangs up:

WARNING[1769][C-0000002d]: sig_analog.c:3095 __analog_handle_event: Ring/Off-hook in strange state 6 on channel 1

Any help would be much appreciated. Thanks in advance.