why is it that i do not need to dial area code 02 if i'm calling within the same area code in manila?

Answer 1

0 is the national direct dial code, 2 is the city code for Metro Manila. The telephone network (due to the way they dole out telephone numbers) is smart enough to figure out that you're dialing a local number vs a national or mobile number.

Be thankful that you can still dial 7 digits to reach someone in the same area code. A lot of telephone companies in the US have converted their switches to require you to dial the city code plus the phone number all the time since those city codes that had been reserved are now being doled out as telephone number prefixes for cell phones, pagers, VoIP, etc.