Do you have a router as well? For the phones, why not just plug the master phone in to the downstairs socket ant the second, wireless , one upstairs with the charger plugged into a 13 amp socket. Leave the existing cable alone, or use an insert that gives you 2 outlets from the master socket, one for the new downstairs phone and the other for upstairs if you change your mind. Don’t forget splitter for router if you have one
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