Old style technology...

...our cord - less phone, which is our landline...

One of the electric cords at the back leads to the plug, the other leads to our router, so we're already on "BT Digital Voice" ( other VOIP phones are available... )

The downside is that it won't work if there's a power cut, so it's pretty useless as a back-up. As hardly anyone uses it to call us, we're now contemplating getting rid of it completely - we both have a mobile phone, so there doesn't seem much point keeping it...