Signal is a popular encrypted messaging application that focuses on privacy. It is an alternative to WhatsApp, Telegram, Facebook Messenger and more. If the app is one of your favorites and it really suits you, it can be set as the default SMS messaging app on your Android device. Unfortunately, the functionality is not available on the iPhone.
Not only will you be able to communicate with your Signal contacts, but you will also be able to send and receive text messages with your phone number. All your conversations can be in one place. Let's see what exactly you need to do!
Select "Settings" from the menu.
At the top of the Settings menu, tap "SMS and MMS".
Then you will see "SMS Disabled" at the top. Select it to continue with its default.
A pop-up will take you ask to select the default SMS application. Select "Signal" and press "Set as Default".
Are you ready! Sending an SMS is the same as sending one message in Signal. The Signal contact list will display Signal people at the top in blue.
If for any reason you want to send an SMS to a Signal contact, you can do so. Get started by typing a message as you normally would.
This time, instead of pressing the send button, press and hold it.
You now have the option to go to "Insecure SMS". As mentioned earlier, SMS messages do not encrypted such as Signal messages.
The send button will now be gray with an unlock icon. Press to send the SMS.
All the necessary settings have been made! Now you can keep all your conversations, whether they are Signal or SMS, in one place. Keep in mind that you will not be able to have access to SMS via the Signal Desktop application.
Source of information: howtogeek.com