Text Messaging
As we have noted elsewhere...Text Messaging cannot be considered to be a "Warning" system.
Most of us have had the experience of not getting a text message from a friend or family member, and then it shows up later, sometime after we've turned our smartphone off and then back on and it "re-associates" with the phone system and any messages piled up in the provider's system are then pushed out to our phone.
Only one "text messaging" system can be considered a "Warning" system, as it bypasses that and goes directly to your phone and sounds an alert tone - and that's the Wireless Emergency Alert (WEA) system, which is described on the IPAWS & WEA page of this website.
However, not all smartphones are WEA capable, so even that's not totally reliable.
Apps
There are a number of companies and organizations now selling or providing free Apps so you can receive Warnings and Alerts. These cannot be considered true "Warning" systems, either as they may face the same reliability issues as Text Messages, your cell phone carrier, WiFi access, and if that system is actually being used in deliver alerts in this area. Even if they promise "nationwide" coverage...remember, you'll only be getting notices that reach their system.