23 housing refers to 23-hour lockdown, which is another way of describing segregation or the Special Housing Unit, commonly known as the SHU or the hole. The name comes from the reality of what it means day to day. The inmate is in their cell for 23 hours out of every 24, with one hour out for a shower or limited movement depending on the facility's specific protocol.
This placement outside of general population happens for several reasons and not all of them are disciplinary. The most common categories are disciplinary segregation, where an inmate was written up for a rule violation and is serving the consequence. Administrative segregation, where the placement is based on safety or management concerns rather than a specific infraction, including protective custody for inmates who have requested protection from threats inside. Or investigation holds, where something occurred and the inmate is separated while it is being sorted out.
None of those situations are good in the sense that 23 housing is never comfortable or easy time. But there is a meaningful difference between being there as punishment for something he did and being there for protective or administrative reasons. The circumstances matter for understanding how serious the situation is and what it means for his overall record.
To find out more, call the facility back and ask to speak with a counselor or case manager rather than whoever answered the general line. Ask whether the placement is disciplinary or administrative. You will not always get a detailed answer, but asking the right question to the right person sometimes yields more than a general inquiry does.
In the meantime, mail still reaches inmates in 23 housing even when phone access is restricted.