。
I wrote her phone number down in my pocket notebook.
Literal
I [topic-は] her [attributive-の] phone-number [object-を] pocket-notebook [to-に] wrote-down.
書きつける ('to write down, jot down') is a compound verb from 書く ('write') + つける ('attach, put on'). 手帳 ('pocket notebook, day planner') is a compact notebook often carried by Japanese professionals and students for jotting down appointments, notes, and contacts.