“Add Wi-Fi Calling for Other Devices” Fails to Activate

On an iPhone 8 running iOS 12 on a pre-paid Verizon wireless plan the ”Add Wi-Fi Calling for Other Devices” option (shown in the image below and circled in yellow) fails to activate:


When the activation fails it fails with the following error:


I have tried calling customer service for support but had no answer to my question. Please can someone with knowledge of this situation explain to me why this avaiable service is failing to activate and what action may need to be taken for it to activate? To date, I have had no answers.

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