From the Android Mobile App: Click the More button at the top left of the mobile app. Select Voicemail. Click Folder with Music Icon located at the top of the mobile app. Click the Record button, give the recording a title, and click OK. The system will call your extension. Answer the call and follow the prompts to record your new greeting.
Calling your retrieval number — instead of your home phone number — is a faster way to listen to your messages remotely. You will be prompted immediately to enter your PIN and don't have to wait for your greeting to start.

Follow the instructions and press that number. You will be asked to delete your current Voicemail greeting. Click on the ‘Yes’ button that appears on the screen if you want to remove that. You can save your new recorded Voicemail greeting or can use the default recorded option. From the home screen, select the Phone app. Note: Alternately, you can set up voicemail by selecting the Phone app then selecting and holding the 1 key. Select Keypad, then select the Visual Voicemail icon. Select Start. Select Ok. From any Home screen, tap the Menu key. Tap Settings. Tap Call settings. Tap Voicemail. Tap Voicemail again. View the voicemail number. Press and hold the 1 on your device’s dial pad to call and connect to your voicemail. Press the star * key to access the main menu. Press 4 to access the mailbox options. Press 7 to change the language of the voicemail prompts to either Spanish or English.
One of the best and quick fix to this issue is to turn off Wi-Fi for once and switch to Cellular data connection and then try to change the default greeting to custom on iPhone.