Starting on Sunday, Minnesotans will have to dial new area codes to reach some parts of the Twin Cities. As of that day, the optional grace period for using two new codes is over. That means in order to call parts of the northwestern Twin Cities as far north as Princeton and Cambridge, you'll have to dial 763 first. Calls to the southwest Metro as far south as New Prague will need the 952 area code. The additional codes have been introduced to accommodate an increasing demand for telephone numbers brought about by mobile phones, computer connections, and other communications equipment. Bryce Hallowell is a spokesman for Qwest Communications.