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.