The BT Infinity system uses fibre optic cable, whilst the phone system uses plain old copper wire, so that helps explain why it is working perfectly.
The crackling may be caused by bad wiring, or it could be due to a fault with the Panasonic phone system.
Do you have any other devices connected to your landline at all? (e.g. alarm system, fax machine)
Does the conventional telephone work ok, or do you hear crackling on it as well?
Does the conventional telephone work ok if you unplug the Panasonic system from the telephone line and connect the conventional telephone to master BT socket in your home?
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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.
John, you are very welcome!
Enjoy the rest of your weekend.
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