Complex sentence: Until 1991, the entire region was a single local calling area, and it was possible for anyone living in the metro area to dial a number in the District, Maryland or Virginia with only seven digits.
Split into simpler sentences: Until 1991, it was possible for anyone living in the metro area to dial a number in the District, Maryland or Virginia by dialing the 7-digit number. It was thus possible for telephone customers in the Washington metro area to dial a number within the metro with only seven digits.