As a contact you can be alerted wherever you are and at any time, day or night. The notification is sent as a "critical alert." This means you will receive an alert even if your phone is in focus mode or set to silent. You will only not receive alerts if your phone is in airplane mode or without an internet connection. Since alerts should only be triggered when your contact is in a critical situation, it is not expected that you will be alerted frequently.
Remember, it's ok not to always be able to help! All contacts are notified, so other people will also be asked to help.