This article describes how to receive emergency notifications via text message while an iPhone is on silent or 'Do Not Disturb' focus is active.  This allows on call and emergency operations staff to silence their phone before bed, but still receive alerts.  

Access the 'Contacts' app.  Create a new contact called 'Emergency Alert'.  Then add the following numbers used by our alert system to send text messages, as individual phone numbers:

Scroll down and select 'Text Tone'.

Select a tone suitable to wake you if needed, the longer the better.  Scroll to the top of the tone selection page, and select 'Emergency Bypass'.