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

Dang I thought I knew the answer to this but apparently I dont. Interested in an answer though!