Here's how the problem breaks down: who called her.
Literal
Problem [topic-は] this-way becomes [explanatory-のだ]. That-is-to-say, who [subj-が] her [to-に] phone [obj-を] made [Q-か] [copula-だ].
こうなるのだ ('it comes down to this') sets up a dramatic reveal. つまり ('in other words, namely') delivers the specific point. 電話をかける ('make a phone call') is a standard collocation.