This is likely to be due to your phone’s current set up. Ensure your phone is switched off “silent mode" and its volume is set to at least medium level. This should ensure even the deepest sleeping parents are alerted from their slumber.
For iPhone users it is worth checking your “do not disturb” mode. If activated a crescent moon or half moon will appear in the status bar to the left of your battery life. This means your phone is silencing all calls, messages and notifications.
To switch this mode off simply slide your finger up from the bottom of the screen and tap on the crescent moon icon.