Verizon error code 9001 generally occurs when voice mail apps fail to work. In this situation you may not access your Verizon account properly, so you need to fix this and to do so follow these steps:
• Go to settings of your phone
• Select data server and turn it on
• Go to Verizon account and click on settings
• Now turn on the slider for passing unrestricted data
• Now you will get the voicemail notifications
In any case, if the error gets back, then you should call at Verizon Customer Care and get a complete solution with easy steps.