Here is a good article on standards, based on UK business primarily but it hits home.
Yet the evolution of VoIP will follow a predictable path. It’s all about the money. Because the incremental cost of adding VoIP to broadband is effectively zero, broadband users are migrating to VoIP in droves. For a while, there will be revenues from PSTN gateways but not for ever: the cost of using VoIP on broadband is actually zero, so the existing users will draw more people into broadband subscriptions. Eventually, it will become cheaper for BT to service the remaining PSTN customers with a broadband connection and VoIP disguised and billed as an ordinary telephone. For telephony, the digital switchover will be complete.