Bob Eager wrote:

But the cost of such a service would be perceived as 'expensive' since
it would effectively have to cover rental of the 'line' even without
phone service. Probably a marketing disaster!

I'm not saying it would be a great deal cheaper, but I think whoever
offered this would (for a change) be ahead of the curve, with BT's 21CN
this is effectively the direction they will push people anyway, a data
line that you happen to do your voice calls over rather than the other
way round ...