I was + ~ing + when you called me. 
네가 전화했을 때 ~하는 중이었어.

I was driving when you called me.
네가 전화했을 때 운전하는 중이었어.

I was cooking when you called me.
네가 전화했을 때 요리하는 중이었어.

I was sleeping when you called me.
네가 전화했을 때 자는 중이었어.

I was working when you called me.
네가 전화했을 때 일하는 중이었어.

I was showering when you called me.
네가 전화했을 때 샤워하는 중이었어.

A : I called, but you didn't pick up.
전화했었는데 안 받더라.

B : I was showering when you called me.
네가 전화했을 때 샤워 중이었어.

A : I figured you were busy.
바빴을 거라 생각했어.