But have you considered a directional or stronger external aerial for your DEC phone?
I was going to ask, and still would like to know, do such devices exist? and are they legal?
Then in a quest to answer that question for myself I thought of another possible solution... a dect repeater. They seem to be made by respected manufacturers and gap compatible so I assume they are legitimate, but that may be a bold assumption.
The advantage of extending the dect range rather than wiring, is you maintain benefits such as full intercom between handsets and, if the dect base station has an answerphone, easy 'cancel' of the anserphone by just going off-hook on any handset.