Hello,
This range of devices requires a Dect base which manages the devices.
Then you need a subscription with a Web operator that allows SIP-type connections.
Personally, I use and offer Peoplefone because the subscriptions make it easy and cheap to connect this type of device.
To avoid the Dect base, it is possible to use a Wifi tel which allows you to connect directly to the tel network using Wifi coverage. Granstream offers this solution:
Grandstream Grandstream WP810