Area codes (redux)

I'm gonna post this here so it doesn't get lost. Berkowit made a good point over on TBLA:

Probably when they started running out of area codes with 1 in the middle (only 64 of those available, if the outer digits had to range 2-9) they had to find another way to let the system know when you were going to be dialing a 10-digit number. Starting with a is that very clever way. It could only mean “area code follows, then number”. So you could now use 0s in area codes.

It totally makes sense. So you tell everyone they have to use 1 before the area code in 1990, then you can create 310 (South Bay-LA), 510 (East Bay-SF) etc. and the server knows that it's going to be a ten digit number. He also mentions this:

When you first start dialing, the phone system doesn’t know how many digits you’re going to end up dialing. But, in the early days, they must have chosen area codes that could not be used as exchanges anywhere so the phone system would know after 2 or 3 digits if it was area code (expect 10 digits) or not (expect 7 digits). Using “1” as the middle number of an area code would facilitate that, since 1 could not be either first or second number of an exchange (dating from the old days which required letters – no letter for 1 or 0 on phones).