Can I connect my Konftel 300Wx to a Konftel 300W DECT base?
Yes, you can do that. If you are using the US version you must go in to the service menu* and select US-LEGACY under DECT REGION.

*to access the service menu, press MENU once then the HOLD-button (lower left button) for two seconds and then the LINE MODE (lower right button) for two seconds.