just do what i do.
let it ring off. if its important they will ring back. again let it ring off its inconvenient for you to answer. if its important they will ring back. if they ring at a convenient time, by all means answer - after all they are paying, if not let it ring off.
eventually they will either realise that it isnt that important and stop bothering you/leave a mesage, or drive around to check you are stil alive, and hopefully bring some viscount biscuits (the minty ones).