moto X pure on plan 3.0 and I’m testing Signal and Google Messenger for SMS and MMS. However, the messages are showing up on the phone/default SMS app, if open. But since SMS/MMS is not default, you cannot send using this app. I thought this was strange that SMS/MMS are showing up in both apps.
The default SMS app cannot be disable or uninstall, but should the app permission be modify? i.e. Settings -> Apps -> Messaging -> Permissions -> turn off all permissions (contacts, phone, SMS, storage)