There is another forum thread with the 1035 Voicemail Error that has been marked as solved, where other customers have provided excellent suggestions to try and resolve the error.
Please give them a try, and let us know how they work out for you! If you need any assistance with navigating through the specific steps, feel free to reference our Device Support Portal to review indepth tutorials and step by step processes for your iPhone 8.
Trace, AT&T Community Specialist
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