Why do messages sent on Doro 5030 always get 'text saved but not sent'?

My elderly father has a Dore 5030 mobile phone and recently he has tried to text out but gets the message 'text saved but not sent'.  How can I rectify this so he can send texts out.

Hi Sue. Can you try a simple restart. Maybe the phone needs to refresh the network signal.

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Are you on the best cell phone plan?