Where is area code 138?

There is no area code 138 in North America (country code +1 = USA, Canada, etc.).

If you mean country code +1 and area code 38, you need one more digit:

• +1 385 = Salt Lake City, UT, USA
• +1 386 = Daytona Beach, FL, USA

Other countries may have domestic area codes like 138.

(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 many countries use different prefixes.)