Important: Ensure that the Razer Phone is up to date. Navigate to Settings > System > Advanced > System update > Check for update.
Step-by-step process
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Go to "Settings" > "Sound".
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Ensure that the phone is set to “Ring” and the volume is set to its maximum for testing.

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Ensure that a phone ringtone is set.

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Call your Razer Phone from a different phone line.
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If the issue persists, reboot the device and test the phone again.
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If rebooting your device does not help, back up your phone data and perform a factory reset. See the answers below for more details.