。
She called me terribly late last night.
Literal
She [topic-は] last-night terribly late me [target-に] phone [object-を] applied-and-came.
大変 ('extremely, terribly') strengthens 遅く. 大変 carries a double-edged feel: as an adverb-intensifier it means 'very, extremely'; as a stand-alone exclamation 大変だ! it means 'oh no!, that's terrible!' Context picks the right reading. Note 掛け with the kanji form 掛 — Japanese variably uses kanji vs. kana for かける; both are correct. The compound 電話をかけてくる ('to call [someone toward speaker]') stitches the directional ~てくる onto 電話をかける, marking the speaker as the recipient of the call.