The 958 and 959 prefixes (NPA-958-XXXX and NPA-959-XXXX) are used as test numbers.
958 numbers are used for most testing, 959 numbers are reserved for inter-exchange carrier testing.
959 numbers can only be reached by being pre-subscribed to that inter-exchange carrier or using an access code (101xxxx-1-NPA-NXX-XXXX).
In other words, to dial an AT&T 959-XXXX number, you would dial 1010288-1-NPA-959-XXXX. To dial an MCI 959-XXXX number, you would dial 101022-1-NPA-959-XXXX.
Test numbers tend to start at NXX-1000 or NXX-6900.
“To dial an MCI 959-XXXX number, you would dial 101022-1-NPA-959-XXXX” is incorrect (missing one digit). According to http://www.1010220.com/faq.html “10-10-220 long distance is provided by Telecom*USA, a wholly owned subsidiary of MCI.”