Can you dial your own number to get o your voicemail password prompt?  You may need to press * or # to get to the password entry menu

When I dial 1+my number . Yes I can access to voicemail. But now I couldn’t directly access to my voicemail by voicemail buttons. It only shows the call fail 

Hi @dongmei 

What phone are you using?

Have you tried resetting the network settings on your phone and see if that helps? 

I am using iPhone XR. How to reset network setting ?

Settings > General > Reset > Reset Network Settings

You can reset network settings using the following steps:

1. From the home screen, tap Settings.
2. Scroll down and tap General.
3. Tap Reset.
4. Tap Reset Network Settings

Please note that resetting network settings will also reset the saved wifi and bluetooth connections.

I already did that before 

The voicemail is working now. I just updated my phone. Thanks for the suppport. 

Try turning off your iphone, taking out your sim, turning the iphone bsck on, and the putting the sim back in again