I do not know how to update the PHS phonebook.

Updating the phone book can be performed on the Clerical PC.
In the case of the teacher, I think it will be a virtual Clerical PC, but after connecting the cable, you will have to update it using the application.
At this time, I think that the phone book could be updated on the PC.

Please refer to the following URL for the connection method of the virtual Clerical PC.
(Help desk [tel. 2655] accepts inquiries about office terminals)

https://docs.sic.shibaura-it.ac.jp/jimu/VMWare_for_teachers_ver20160219.pdf