。
I'm at the office because I'm busy.
Literal
Busy-[て] company [at-に] am.
The polite います gives a flat factual report — 'I'm at the office because I'm busy' — without the explanatory のだ that would frame it as 'let me explain.' Compared to a casual いるんだ, this version sounds dryer, more like an answer to a colleague's polite question than an intimate text. A useful illustration of how dropping or keeping のだ changes the affect of an otherwise identical statement.