And that is because the grouping of digits in a phone number influences the memorability. Two groups of 4 digits are no harder to remember than one of 3 and one of 4. And those researchers might be astonished to discover that the Aussie mobile phone numbers are also memorable: and they're of the form 04xx yyy zzz! So much for the "7-digit" rule, as Darrell pointed out.
Wade.