If the role requires you to be on-call after hours, that will be mentioned in the job description, so it should be no surprise when mentioned in the interview. The interviewers want to ensure you read the job description in its entirety and are aware of this requirement before moving along with the interview process. If you want to be considered for the position, you will need to let the interviewers know you are flexible, a team player, and will have no issue with being on-call.