A phone number consist of seven digits plus the area code. How many possible phone numbers can have the area code 229?

That would depend on the restrictions on the number.

i.e. besides the area code, the number rarely begins with 0 and there is most likely no such number as 229-000-0000.

But, that notwithstanding, there are 7 digits to place and 10 choices for each (assuming repetition and no restrictions as aforementioned). So, there are
10^{7} different numbers in said area code.

I'm sure that the OP is looking for the answer as Galactus gave it - after all, this is a "math" forum, not a "telephone company" forum. However, in case you're interested: the actual number of usable telephone numbers for any given area code in the US is actually less than 10^7. Since the first number of the exchange code is restricted to the digits 2-9 (0 and 1 are not allowed), this reduces it to 8 \times 10^6. Then the number of options is further reduced by certain rules such as no phone number is assigned between 555-0100 and 555-0199 (that's why the fictional phone numbers used in movie or TV scripts are always 555-something) - this takes out 100 possibilities. Finally, the second and third digits can't both be 1 (such as 911) - which takes out another 80,000 combinations. After you take these rules into account you end up with a total of 7,919,900 possible numbers within an area code.

I think in the old days, the middle number of an area code was an 0 or a 1.
I see other numbers now.

I think in the old days, the middle number of an area code was an 0 or a 1.
I see other numbers now.

That's right - it changed in the US back in the 80's. The first digit of an area code is still restricted to 2-9. The middle digit used to be only 0 or 1, which let the switching equipment distinguish between an area code being dialed versus the "NNX" exchange code, which at the time didn't alow either a 0 or 1 for the 2nd digits. This enabled 7-digit dialing for local calls to be easily distinguished from a 10- or 11-digit number, even as the dialing was in progress. Now that an area code can "look like" an exchange code most phone systems require you to dial 1 + area code + number, even for local calls in your own area code. Otherwise there would have to be some sort of "time out" mechanism that would guess whether you've finished dialing after 7 digits, and that would lead to more wrong numbers.