8 digits is only 100,000,000 possible unique numbers. So it looks like they're simply running out of numbers.
10 digit numbers are still in use:
When Singapore ran out of phone numbers, they simply tacked on a single digit to every existing phone number. Japan's population is dwindling, that seems like a much easier option.
Sure, but the US with a much larger population only uses 10. 14 is bonkers, especially if it's just to preserve a method that basically wastes 3 of them.
As a user of this numbering system, this is how it generally looks like:
Landline numbers are dialed
0 - prefix to dial outside my area code
XX(X) - area code
XXX-XXXX - phone number
Mobile phones are
0 - prefix
70/80/90 - "mobile phones" area code
XXXX-XXXX - phone number
Since every number starts with a 0, you don't think of it as part of the number. Without the 0, most landlines are 9 numbers long and mobile phones are 10 numbers. When you're calling between landlines in the same area code, you only need 7 numbers.
The largest issue is the death of landlines and the rise of mobile phones. Lots of area codes dedicated to landlines when landlines are dying.
The NANP (north american numbering plan) is basically the only place in the world where cell phones get put into regular geographic area codes, due to the way cell phones are billed there (in the rest of the world, receiving calls on a cell phone is free, the caller pays the extra fees)
Brazil (+55) has mobile in geographic area codes, but separate prefixes within the area codes and longer than landlines (except for Nextel, which is a radio phone, not a mobile phone).
Some countries have been getting mobile phones in landline ranges as tarriff arbitrage, like Germany (+49) and Russia (+7, along with Kazakhstan, but I think it's only getting fuzzy in Russia). Others have been getting mobiles filtering in as fixed CDMA carriers using landline ranges start supporting mobile CDMA handsets (I forget where, but a few countries in Africa and southeast Asia)