Complex sentence: The leading part of this address was the area code (three digits), followed by a seven-digit subscriber number consisting of three digits for the exchange or central office and four digits for the station or line number.
Split into simpler sentences: The leading part of this address was the area code (three digits), followed by a seven-digit subscriber number. The subscriber number consisted of two parts, a three-digit number for each telephone exchange or central office, and four digits for the station or line number.