Complex sentence: When multiple NPA codes serve a calling area in an overlay arrangement, ten-digit (10D) dialing is required, while seven-digit (7D) dialing may be permissible in areas with single area codes.
Split into simpler sentences: When multiple NPA codes serve an area in an overlay arrangement, ten-digit (10D) dialing is required. Seven-digit (7D) dialing may be permissible in areas with single area codes.