After changing from an Apple device (iPhone, iPad, etc.) to a non-Apple device (Android, Windows, BlackBerry, etc.), and keeping the same phone number on the new device, SMS messages are no longer received from Everbridge.
If a contact has changed from an Apple device to a non-Apple device and still has the same phone number but is no longer receiving SMS alerts, it is likely a problem with iMessage on the contact's Apple device.
Deactive iMessage on the contact's Apple device. The following article explains how to deactivate iMessage: https://support.apple.com/en-us/HT204270.