Where in the US is the telephone code 00123?

That's not a valid number, as dialed from the United States, because "00" is a standalone short code for reaching the long-distance operator.

Much of the rest of the world dials telephone country code +1 for North America (USA, Canada, etc.) as 00 1, but you then need the following three digits to narrow down the location.

• +1 231 = Traverse City, Michigan, USA
• +1 234 = Akron/Canton/Youngstown, Ohio, USA
• +1 236 = British Columbia, Canada (overlay on the entire province)
• +1 239 = Ft. Myers, Florida, USA

Any of those numbers would be dialed as 1-23X-xxx-xxxx from the United States.

(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.)