The interviewer would like to see what your availability is and if it will fit the needs of their organization. Being on-call as a biomedical technician may be necessary, depending on the size of your team. If a piece of equipment fails in the middle of the night, a doctor does not care what time it is. The patients' health is at stake.
Assure the interviewer that you, and your family, will be okay with you leaving to work in the middle of the night from time to time. Also, take this question as an opportunity to ask how often you would be expected to be on call. Be careful not to rule out the position entirely before finding out all of the details.
"My family understands the potentially late or random hours required of a biomedical technician, and we are all prepared for that. In my current position, I work on call one weekend per month. Could you share your on-call schedule with me?"
"I understand that being on-call is a requirement for this role and I am willing to participate in your on-call schedule, as required."