At one time, the 848 area code was the only area code that was needed in the state of Oregon. That meant it covered the entire state and there were plenty of phone numbers to go around. When cell phones became more popular, people found the need to have more than one phone number. That put a strain on the system, and a new code was added to the state. That code was 541, and it split from the 503 code. After that, it became clear that an additional code was needed, and that was the area code 971.
When the area code 971 was introduced, those in the 503 area code had a new set of numbers to dial. That meant some had a different area code for their mobile number and their landlines. This overlay was all of the 503 code, with the exception of Tillamook and Clatsop counties. They remained exclusively with the 503 code. This should be changing in April of 2008. That means the entire area of 503 will be overlaid by the area code 971.
Though there is some confusion when you have overlaying area codes, you can be sure any phone number with the area code 971 is coming from the northwestern part of the state of Oregon. That includes the cities of Portland, Salem, and Astoria. This is a small area, but has the highest population in the state. The 971 code is not used as much as thought, but some unknown calls coming into your home may have this code.