The adapter is a generic part, RJ11 plug to BT socket adapter. Readily available from your preferred online supplier for typically sub-£5.
If you have a relatively new phone then the most commonly-available version(s) should work. If you have an old phone (typically >15 years old, early 2000's or before) then you might need an adapter which has a 'ringing capacitor' in it (which allows an old phone to produce a ringing tone).
Sometimes the forum team can arrange for a passing technician to drop one off through the letterbox (usually for new customers though). One of the forum team will reply in a day or two if you want to wait for their response.
