Openreach and KCom (in Hull) are the only two infrastructure operators deemed by the regulator to have 'significant market power' and so are under a general obligation to open up their network on a wholesale basis for others to provide retail services over the top.
The only circumstance in which any other provider would be under a similar obligation is if they have received direct subsidy (state aid) from UK government under the old Superfast programme or now Project Gigabit.
An alt-net which has entirely privately financed its fibre rollout is free to pick and choose what retail services it offers. It may make a decision to open its infrastructure on a wholesale basis to anyone else, or only offer its own captive retail products - that is a commercial decision. BT, Plusnet or any other retail internet service provider doesn't have an automatic right to offer their services over an alt-net's (competitor's) privately financed infrastructure, and customers need to be aware of this.
Which shows OR isn’t as independent as Ofcom wants it to be.
I think that is a misreading of the market situation.
If I don't want BT/Plusnet digital voice but say want to move my (soon to be) IP landline voice service to another provider, like A&A how easy is it, without cutting off the Plusnet broadband service (which I am ok with)?
If A&A (assuming they are not in receipt of government subsidy, which is highly likely to be the case if there is already service from Openreach) have made a decision not to open up their network to an ISP who they see as a competitor, that is their commercial prerogative and your choice will be confined to those ISPs that they allow over their network.
If A&A have opened up their network for any other ISPs to provide services over the top, whether by choice or because they have received government subsidy, there is no obligation for any ISP to take up that offer of access if they deem the faff and expense of integration and interoperability as not being worth their while commercially.