The only connection is via the telephone port at the back of your broadband Hub. Extension phones aren't connected.
The simplest solution would be to buy a DECT cordless phone kit - they're not unduly expensive. The kit comprises a base station which plugs into your broadband Hub and one or more handsets which can be placed wherever you wish, or carried with you. The handset(s) connect cordlessly to the base station to make and receive calls..
