Question

Yo me duchaba cuando ___________ el teléfono.
1. sonó
2.sonaba

Answer 1

sonaba is what you want

Answer 2

The answer is sonó

Yo me duchaba cuando sonó el teléfono.

The verb duchaba is in pretérito imperfecto and matches with simple past (in this case sonó). This pretérito imperfecto describes an action or state in the past, without indicating whether it was completed or not.