Thanks for the detailed response, John.
You can ignore the automated offer of remote assistance, it is sent by the site automatically and is not relevant to your question.
I've been looking further into the phone system to find out what the '25' might mean - it is not an error code, so it indicates something else, perhaps the number of missed calls or the number of voicemails waiting to be listened to.
If you could attach a photo of the display I might hazard a better idea as to what it means.
As the conventional phone is also affected, this is likely a line issue. First, check all internal wiring to make sure none is damaged or badly connected if anything was moved recently, or if pets might have gnawed the line, etc.
If all checks out ok then it will be up to BT to find where the line fault lies - it could be that floodwater has gotten into an underground cable somewhere, or a tree branch is touching an overhead wire, or something similar is causing the crackling on the line.