SOTAP is for lines that DON'T also carry broadband, AND are in areas that don't have VDSL or FTTP.
For existing VOICE ONLY customers with analogue phones currently connected to the local telephone exchange
SOTAP will use a dedicated ADSL connection back to the exchange to emulate an analogue phone line
this is referred to as a 'pre-digital' landline phone.
This is a temporary measure to allow Openreach more time to provide universal FTTP coverage, and upgrade those customers to VoIP over FTTP, before the telephone exchanges are decommissioned in 2030 and all SOTAP transitional products are withdrawn.
The problem with VoIP and QoS on ADSL, is that you need a theoretical minimum upstream available bandwidth somewhere around 680Kbps to 720Kbps, for there to be enough packets passing through the device performing QoS, to mathematically have any chance of prioritising and rearranging the outgoing packets for VoIP data in real time, while simultaneously using the broadband for other things.
Many people still on ADSL are in remote areas, on very long lines back to the exchange, and are unlikely to be achieving those minimum upstream speeds, so no amount of QoS will provide usable VoIP calls unless they completely cease using the broadband while making calls using VoIP over ADSL - very inconvenient for incoming calls !
For customers who get the 1.0Mbps to 1.2Mbps upload speed of ADSL2+, then yes an ISP supplier router with QoS would probably enable VoIP to work, but you can't blanket say that VoIP will work over ANY ADSL line if you use a QoS capable router.
FWIW, for several years I successfully used VoIP over "ADSLMax Premium" with 832Kbps upload sync, 720Kbps usable upload bandwidth, so yes it is possible, but spent a lot of time building my own QoS server to achieve it, and experimenting with the limits of the minimum bandwidths that were required.
For existing ADSL (i.e. broadband+voice) customers, there is a problem with the PSTN switch off.
Plusnet support when asked, have stated that Plusnet won't be supporting SoADSL lines (like most other ISPs).
Therefore when Openreach unilaterally switch off the PSTN voice circuits, the Plusnet ADSL lines will fall back to be SoADSL, and therefore Plusnet customers will lose broadband, phone, and anything else associated with their accounts.
Currently, those abandoned customers would have to move to one of the few (and expensive) ISPs that still sell SoADSL.
However for the reasons above in the QoS discussion, if the SoADSL connection isn't capable of supporting the minimum upload bandwidth for QoS to prioritise VoIP ahead of other broadband traffic, then the only options to have a VoIP phone are to have a second ADSL line that provides ONLY VoIP traffic, or somehow persuade Openreach to provide a second dedicated SoTAP line to provide the essential landline phone (although so far Openreach say that SoTAP is only for existing voice only lines).
