Oh no, I am sorry that you are having this issue for that amount of time. All I do is text message so I understand how important this is to you. What is the exact model iPhone you have? Is your phone and all other installed apps up to date on their software? Have you ever been able to use this application on this phone? Is there a specific reason why we are wanting to use this app versus the preinstalled iPhone message app? Here is a link that will give you more information on set up and use of that app http://vz.to/2snW4Rg.
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I have the same issue for over a year now. Trying to get messages from my basic phone on my ipad mini 4 usind verizon message plus. I’m always getting the unknown error after inputting the code sent to my phone.