Do you use any third party ‘cheap rate’ call providers , using some sort of access code , or call using BT at BT call rates etc ?, even if you don’t , the fact that mobile calls are also affected sort of eliminates your landline end….if you use BT , you could try registering with a call carrier and try a call using them to see if it’s the same , or if you use a third party for international calls , try BT or a different call provider .
Issues like this will prove to be impossible to trace and fix using regular reporting methods , but it’s possible the issue could simply resolve itself if the system at fault , whoever it belongs to , in this county or in France , is fixed because it has self reported, or is picked up by routine testing.
