It's no longer optional-- you now have to dial two new area codes to reach some parts of the Twin Cities. The new codes became mandatory yesterday, about ten months after they were first introduced to accomodate an increasing demand for telephone numbers. Now if you're calling parts of the northwestern Twin Cities as far north as Princeton and Cambridge, you'll have to dial 7-6-3 first. Calls to the southwest Metro as far south as New Prague will need the 9-5-2 area code. Officials from Qwest Communications have said this switchover is one of the most technically challenging splits ever done in the U-S. Bryce Hallowell is a spokesman for Qwest communications, and he's on the line now.