The TextNow application will attach a small notification stating "message failed" with your texts that fail to send. If the app has trouble connecting to the wifi it will display a red banner that says network error, please check you internet connection.
If these are not present, and you have access to the web on the phone your messages should be received and sent. If you find that this is not the case you may need to visit the app store and update TextNow to the latest version.
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