Digital Voice does not use Wi-Fi. It uses a different type of wireless signal called DECT. This remains on even when the Wi-Fi is turned off.
Edit: Typing at the same time as Imjolly.
The SH2 must be the first thing connected to the ONT (for Full Fibre) or the wall phone socket (for FTTC/VDSL).
If your "digital phone" is a traditional cordless unit then the base station needs to connect to the green phone socket of the back of the SH2.
If it is a non-cordless, ordinary phone it also needs to plug into the socket on the SH2.
If a BT digital unit it will need registering to the SH2 as base station. This is in the management pages of the SH2, as is the place to turn off the Wi-Fi.
If Digital Voice has just gone live, be aware that the SH2 will need a reboot before it picks up the correct settings.
Digital Voice is a proprietary version of VoIP, although still SIP based. I doubt that phone will work but others on here know more about this area than me, so someone might know a way.
As far as I know, only BT's own digital phones can be registered direct to the SH2. As I said, it's a proprietary system.
I'm in the same predicament as you.
Not sure if the following will help anyone, but thought I would share my solution that is a little different and has a good degree of flexibility. We have recently upgraded to BT Digital Voice with a SH2 and have a SIP phone system running Avaya IP Phones through my IP PBX - YeaStar S20 through out the property.
I installed the IP Phone system well before the switchover was started because I wanted hard wired phones and the ability communicate between our house and our home office. When looking to upgrade to DV I wanted a simple upgrade path that did not involve buying a whole load of new or replacement kit.
My solution was to connect the YeaStar S20 (IPPBX) to the green phone port on SH2 (in place of the analogue wall socket) and continue using my IP Phone system without any change. My IP phones are connected to my LAN via Cat6 cabling that I installed. This meant that all the settings in the IPPBX remain unaltered and all the scam call blocks etc I had set up operated seamlessly. It also means that because I have a UPS installed the phone system will not be affected by a power outage at home.
This might not be for everyone, but having an IP phone system through out the property (eliminating DECT call drops/batteries dying mid call etc) allowed me to retaining a hard wired flexible system that also allowed a very simple upgrade path for DV. Although the Avaya phones can all connect using WiFI, I chose to hard wire then via my ethernet infrastructure.
The advantages are that every room can have a hard wired IP phone connected on the LAN (and therefore be able to reach any other phone on the same network); for those without ethernet cabling, using WiFI SIP phones would be an alternative, though you would need to use a small IPPBX and WiFi Access Points as necessary.