Digital Switch Over Mess

[#1]

I strongly suspect that there is no business case for supporting SoADSL - the cost of providing it (for a few years until ADSL is retired altogether), and the overhead of updating internal systems and training support staff to deal with it, probably makes it uneconomic.  
Added to that is that the impact on average broadband speeds used in advertising - ADSL really brings down the average, even if only a small percentage of customers have it.

Passing these cases over to BT for SoADSL or EE for 4G/5G (or just walking away) is probably the most sensible approach from a business perspective.

Of course it isn't good for customers with only ADSL available.  

[#2]

@corringham 

A very valid sound argument, but not an excuse for reporting an "error" in place for "we cannot offer service to your premises" when users look for service availability.

[#3]

I agree with that.

If I were Plusnet I'd use a message along the lines of "We can't do this online - call us on <special number> to discuss the best optons"

And I'd have a special contact number that connects to support staff that know about SoADSL and alternatives such as 4G/5G that BT and EE can provide. Of course, Plusnet would still lose the business (so from a Plusnet business point of view it is no better than the error message) but some business may remain in the BT group.