Greetings! Thank you for asking about your reset options. By choosing to "Reset All Settings" on an iPhone 4, you will be putting the device under a factory reset which will purge all personal data. This is typically the last step you may need to take in resolving a device issue. I only recommend it if you've exhausted all other options.
I see that you're trying to resolve a Visual Voicemail issue. If this has not yet been cleared up, please let me know as much detail about the issue that you can share so that I can help. Thanks!
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