As soon as you think you may not be able to make your monthly service appointment you need to call your ignition interlock manufacturer, not your service center. They can advise you of a new time to come in for service.
If you are traveling, or otherwise unable to bring your vehicle to the service center, you may have a friend or family member bring it in for you.
It is important to know that you must have your device serviced every thirty (30) days or more often. If you go more than thirty (30) days between services you may be extended on the program.