How to Find a Dummy Phone Number to Use for Test Data

According to the commentary site, there seems to be no "future non-existent phone number" on the 0120 or 0800 incoming billing phone numbers (so-called toll-free numbers).

From the Telecommunication Numbering Status of the Ministry of Internal Affairs and Communications,
2. Additional Service Phone Number → ·Incoming Billing Function list also shows that 0120-000-GHJ to 0120-999-GHJ and 0800-000-GHJK to 0800-999-GHJK are all in the table.(There is an unscheduled additional service identification number frame as described below.)

If there are no vacancies in the above range, you should be able to read Telecommunications Number Plan (Ministry of Internal Affairs and Communications Notice No. 6) PDF p6. Additional Service Phone Numbers in the Third User Equipment Identification Number table.
However, the unreserved number bands in the table (for example, 0120-902-XXXX) appear to be numbers that do not exist as of September 30, 2021.

In addition, as the answer text is an expression of hearsay and estimation, I am an amateur as a respondent.
I would like to emphasize that this may be a real number due to an error in this answer or the additional service identification number in the future.

What is the purpose?

Random numbers are sufficient for test data such as dummy user information that is not allowed to use actual data.Like your first name, it can be coincidental with a real phone number, but that's not usually an issue.

If the test data should be used for tests that should not be sent to a real number, such as over-the-counter or SMS transmission,

• Test with random numbers in closed environments (never call outside)
• Test with your own phone number in a real environment

You need to think about how to test data by changing it in two stages, such as

If your company's rules require you to use data that is non-existent, you may want to ask the department that sets the rules to create test data that will ensure that it is OK.