W80DM W57R Line issue

[Boxtec]

Hi, we've just purchased three W57R's running via a W80DM (and a W80B for range) which replace an old W52P setup.

Within the W52P admin GUI you used to be able to assign numbers to handsets - we used to have three extensions from our 3CX system which we could assign to each handset and choose which was the default extension.

Looking at the W80DN admin GUI I cannot see this functionality now exists, or if it does perhaps I've missed it? What we want to do is recreate what we had on the old W52P setup where we had two handsets on the same extension which rang at the same time as calls came through.