Although I agree that BT as your provider have a responsibility to you , your relatives that are Sky mobile customers should be thinking that Sky have a responsibility to them, in other words your ‘complaint’ , that ‘Sky customers cannot contact my landline’ , is no more valid than the Sky customer stating ( to Sky ), ‘ I cannot contact a BT landline ‘ and you state that simply was dismissed by Sky as ‘contact BT’ , presumably you at least concede BT haven’t taken the apparent Sky attitude of saying ‘ Sky problem, contact Sky’.
If a letter were posted to you from ( for example ) America, but did not arrive, immediately assuming that the letter delivery must have failed in the UK , because the American carrier said so , could be a little naive, did the letter ever leave the American ‘network’ and make it into the British one , you cannot deliver something you never have possession of.
Interconnectivity between networks , landline , VoIP , mobile is complicated and ( obviously ) requires multiple provider ‘inputs’ when calls transit across disparate networks.
Sky mobile is a MVNO , basically Sky use O2 , they don’t have their own mobile network, even if your issues started when you were converted by BT to DV from PSTN , it’s possible that Sky /O2 have network/data changes to make in their own network but it’s also possible that Sky/O2 have done whatever is necessary, (which may be nothing) , and the problem is entirely within BT , but unless you have an intimate understanding of the way these interconnections are handled , it’s pure speculation where problem is .
There have been examples, not involving DV , where calls from everywhere to a BT customer are successful , apart from one network provider ( that may encompass more than one provider , like Sky and O2 , and any other MVNO on O2 ) but BT offer a penalty free exit , not as an admission that they are at fault , but accepting that BT cannot effect data changes in other operators networks, and if this other network insists there is no error and doesn’t cooperate in investigation where the error is , there isn’t much BT can do about that, and at least are not holding onto a customer that has a known issue that proves to be ‘impossible’ to resolve.
I must stress, I’m not blaming Sky , I’m simply suggesting that as we have no way of knowing the intricacies of network routing , we cannot really apportion blame.
