I would guess most of us will try and get the issue resolved by the ISP before going down the Ombudsman route https://www.ombudsman-services.org/sectors/communications though some customers are the bolshie type and will use this rather than letting their ISP try and fix it.
Or don't understand what they are complaining about
The last couple of faults I've had have been outwith my ISP's control. OR network issues, however my ISP did fight my corner very well.
In in some instances you could get the bolshie types complaining about the ISP when in fact they have no control. For example a bad port and pair in my case.
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