because I got a call전화가 오다 literally means “a phone call comes” but naturally means “to get a call.” 와서 connects this cause or preceding event to what follows, so it can also mean “I got a call, so…”
KO: 전화가 와서 잠깐 나갔다 왔어요.
EN: I got a call, so I stepped outside for a moment.