phone-number = ["("] area-code sep office-code sep subscriber
area-code    = 3digit                       ; 3 digits
office-code  = 3digit                       ; 3 digits
subscriber   = 4digit                       ; 4 digits
sep          = *3(%d32-47 / %d58-126 / %d9) ; 0-3 ASCII non-digits
digit        = %d48-57                      ; 0-9