We'd love to help with this. Low or intermittent coverage could be the cause. If you're noticing only one bar or if your device is otherwise moving slowly in regards to data, that would be it. Otherwise, this is possibly a sync issue between iMessage and Facetime. Go into Settings, select iMessage and Facetime, and toggle each off. Afterwards, power down your iPhone completely for thirty seconds before turning it back on again. Turn each back on and let us know if you're still receiving the Texts from Unknown error message.
Have a great week!
James, AT&T Community Specialist
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