Connecticut is adding a new area code to keep up with growing demand for telephone numbers. The state Public Utilities Regulatory Authority announced Monday that 959 will be assigned where the 860 area code is used in northwestern, central, and eastern Connecticut. Regulators are responding to demand as the number of cellphones soars. Remaining unassigned numbers in the 860 area code are expected to run out by year’s end. Regulators said the 959 area code will go into effect Aug. 30. Customers requesting new service, another line, or seeking to move phone service may be assigned a number in the 959 area code.