If there is only one socket and it is the master socket then it is down to BT, assumng you have tried a different phone. You don;t need to change it for a newer one and see if it work sany better as BT are the only ones who can replace them, or that's what they tell you, so it's up to them to change it for a new one, but it doesn;t really matter as there's no extensions so no point.
As for the orange\white ring wore, not many new phones use the ring wore to actually ruing, most modern phones use 2 wires and use internal electronics to provide the ring tone so no current drawn. Also with ADSL all filters are 2 wire only and have their own internal ring capacitor so a failing master socket capacitor would make no difference (in this instance, assuming ADSL is on the line).
Basically, other than the phone itself there is nothing inside your house that is not down to BT to fix or maintain so as long as you are 100% certain the phone is fine and you only have a master socket on the line then get BT out.