Parkend Ring back Circuit
When the caller clears relay L releases. L1 operates H. H2 and H3 connect ringing and ring return battery to the callers line. The caller's bell rings. Relay F does not operate to the AC current flowing round the line to the caller's bell capacitor.
When the caller lifts his receiver and loops the line relay F operates to the DC flowing round the loop. F1 releases relay B. B3 removes the earth from the P wire and releases the callers line. The caller then receives dial tone. B2 releases relay H slowly and the loss of the loop releases F slowly. H1 restores the battery to the P wire to indicate that the relay set is again free.