You can send SMS even if your app isn't the default SMS app.
However, you won't be able to use the SMS provider.
That's the whole point of the new version - to make it clear to the user which app is allowed to use special SMS operations and have only one default app for this.
look at this text (taken from android developer blog) :
In consideration of some apps that do not want to behave as the
default SMS app but still want to send messages, any app that has the
SEND_SMS permission is still able to send SMS messages using
SmsManager. If and only if an app is not selected as the default SMS
app on Android 4.4, the system automatically writes the sent SMS
messages to the SMS Provider (the default SMS app is always
responsible for writing its sent messages to the SMS Provider).
so, in short, if all you need is to send and receive SMS, you can still do it.
however, some operations require that you will have your app as the default SMS app.
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