In my case, the two phones are about 10 metres apart. Since there are a few walls in between, I had to install a Gigaset repeater to amplify the signal, but now it works fine.

I have one unit on the ground floor and the other on the first floor and the connection works fine.

The range is perfectly adequate in a large flat.
The 2 phones connect to the base using the DECT standard. It is also possible to communicate between the two devices.