I have 4 BT8600 phones and recently all the ringtones have failed except on one which still works.
I checked all the settings on all 4 phones and they were identical.
I sent a query to the BT helpdesk and received the following reply.
To Restore Ring Tones on Handset.
Reset the handset settings
I followed these instructions and all phones now have their ringtones back and working.
Go to Solution.
It's good to see you back on the community and thank you for posting.
That's really useful information for anyone else facing the same problem.