In telecommunications, a plant test number is a telephone number reserved for use by telephone installers or other workers to test an individual telephone line.
Test numbers are typically unlisted.
The North American Numbering Plan (NANP) reserves the exchange (central office) prefixes 958 and 959 for plant testing.
[1] Central office code 958 is intended for communications between switching centers.
Central office code 959 with three or four additional digits is dedicated for access to office test lines in local exchange carrier (LEC) and interoffice carrier (IC) central offices.