4) Believed that people can remember a 7-digit number - they can't, unless it is one they use regularly
Really? - Numbers here in Denmark are 8-digit and I remember most of the numbers in my contact list and often dial them directly instead of using the contact list.
Oh, and US numbers are actually 10-digit, but for most local and semi-local calls the 3-digit area code can be omitted.
We used to have the same kind of area codes in Denmark (6-digit numbers and 2-digit area codes that could be omitted on local calls) but about 20 years ago it was decided to throw away the disposable area codes and merge them with the phone numbers, making them always 8-digit. This also allowed for portable numbers that could be moved all over the country, from provider to provider and of course from fixed line to mobile and back.