@Vjn
As @Mark39 indicates, 'BT Digital Voice' branded hardware supplied by BT when they are the ISP will only work on BT Broadband with a BT Hub. The handsets cannot be transferred to any other ISP, which is unfortunately something the ISPs aren't great at telling users when they 'switch'.
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Sky Glass 55" (on ethernet) & two Stream Pucks (one ethernet / one WiFi)
BT Halo 3+ Ultrafast FTTP (500Mbs), BT Smart Hub 2
