The song is by Sydney Carter. It's not called The Telephone Song, as on the thread linked to the thread that's linked to the above thread(!), but Say Who You Are, Love. It's in one of the 4 Stainer and Bell books of Sydney's songs, In The Present Tense. The Stainer and Bell website has all the details. There's a lovely recording of it on Sydney Carter and Jeremy Taylor at Eton (Sydney ably accompanied by Martin Carthy), an LP which will, alas, probably never make it to CD.