Where in North America is area code 308?

Area code +1 309 is used in Bloomington, Peoria, and Moline, Illinois, USA. It includes the Illinois side of the "Quad Cities" metro area of Iowa and Illinois.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)