Residential Digital Voice uses a proprietary system baked into the SH2. That requires either BT's own DV handsets or a regular analogue handset plugged into the SH2. You can't use an IP handset with residential DV.
If you have a different VOIP provider then you'd need to ask them about config.
If you're a business account holder you need their forums, as the cloud based system is different to residential.
https://business.forums.bt.com/t5/Phone-system/bd-p/viop
