As you know 았/었어서 is an awkward expression.
When someone uses it, he/she might want to emphasize that the first action was completed before the second action.
But, it still sounds awkward to most people.
그때 휴대폰 수리를 맡겼어서 전화를 못 받았어 - awkward
그때 휴대폰 수리를 맡겨서 전화를 못 받았어 - natural