Surely they're offering full spectrum audio now...?
BT have totally failed to say they'd be able to offer much better audio quality on a decent VOIP line rather than the 'select' frequency range we've suffered until now. This is the best opportunity to upgrade call quality since phone lines became commonplace last century.
Anyone who's compared the audio quality of a Skype* call to the fixed or mobile standard realises it's comparing two tin cans & string to studio monitors.
Plus, if all the VOIP handsets can't handle that quality audio then they've an excellent opportunity to partner with ones who can.
Anyone would think the commercial opportunities here have been deliberately overlooked just to reduce their ongoing PSTN costs - very short-sighted!
* caveat: over a decent internet connection.