The answer is <u>sonó</u>
Yo me duchaba cuando <u>sonó</u> 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.
It would be sube without the “n”
The answer is:
No, yo no se
