Beginning in April, many new outer-borough phone numbers will be assigned area code 929, adding to the city’s existing 212, 917, 646, 718 and 347 codes. Over seven million 929 numbers will go to landlines and cellphones, as 718, 917 and 347 numbers will be used up by next fall. The North American Numbering Plan Administration, which oversees area codes, notes that whereas previously most families had only a single phone line in the home, now most children have a cellphone, thus necessitating adding more numbers.