I don't see why it should change. BT sells services to residential users, Openreach does not. BT also took the broadband USO obligation, not Openreach.
Like many, I find it hard to reconcile the simple "BT" with the various divisions, and the Group, and I thought it was group that was the legal entity "British Telecommunications plc".
It means BT is free to use non-Openreach services (such as 4G) to supply USO.
Makes a lot of sense, especially as they own EE.
Arguably the wording around "Public Telephony Network" might need to change in that document, given that the wired PSTN is being switched off; but in a wider sense the Public Telephony Network will still continue to exist. That is, people can still make phone calls to each other, there's a managed numbering plan etc.
I always though PTN was the integrated network including international links, mobile systems, and VoIP, so I guess the lawyers were ahead for once!
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