T-Mobile is warning customers in area codes 360, 425, 253 and 206 about the upcoming change.
The new year will also bring a new area code to Western Washington.
T-Mobile began warning its customers in area codes 360, 425, 253 and 206 about the addition of area code 564.
The 564 will be an “overlay” area code and will be issued throughout the region when current numbers run out.
The North American Numbering Plan Administrator projects the 360 area code will be out of numbers by early 2018, according to the Utilities and Transportation Commission, which approved the new area code earlier this year.
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To implement it, the commission also approved mandatory 10-digit dialing for all Western Washington area codes by the fall of 2017. This will require callers to dial all 10 digits, including area code, when making local calls.
T-Mobile said in its notice to customers that new numbers may be assigned the 564 area code beginning Aug. 28.