Complex sentence: In 1999, the entire area 360 was to be overlain by 564, but the implementation has been delayed indefinitely by order of the Washington Utilities and Transportation Commission, which currently forecasts that supply of 360 numbers will be exhausted in 2017.
Split into simpler sentences: In 1999, an overlay area code of 564 was to be introduced to the entire 360 area; however the implementation has been delayed indefinitely by order of the Washington Utilities and Transportation Commission. Currently, the Washington Utilities and Transportation Commission forecasts that supply of 360 area code numbers will be exhausted in 2017.