Android
In addition to supporting end-to-end encrypted messages and calls with other Signal users, Signal Android can be configured as your default SMS/MMS app. In this case, you can send and receive insecure SMS/MMS in Signal Android. Learn how to distinguish when communication is private (Signal message) or insecure (SMS/MMS).
My Signal contact is offline, how do I send an SMS?
You can tap and hold or long-press the send button on Android to switch to an insecure SMS which will have a grey send button 
.
My SMS/MMS is not sent, what do I do?
Text messages sent through your mobile SMS/MMS plan are insecure and need your phone to be connected to your mobile network.
- Confirm you have SMS (text) or MMS (Group and media) as part of your mobile plan and that you are connected to your mobile network.
- Enable Signal Settings
> SMS and MMS > 'WiFi Calling' compatibility mode and try again. - Confirm all the application permissions are enabled for Signal.
iOS
All communication through Signal iOS is private. Apple does not allow other apps to replace the default SMS/messaging app.
Desktop
Signal Desktop does not send or receive SMS/MMS messages. Only Signal messages will be sent or received. The desktop app is an independent client that works whether or not your mobile device is present or online. We also want to encourage users to move away from insecure legacy protocols.