I can't check my voicemail on my Huawei P20 Pro Android 8.1
Cause 2 of 5: You're trying to check your voicemail from another phone than your own
The default voicemail number can only be used if you're calling form your own phone.
Solution: Call +447836121121 and key in your 4-digit PIN to check your voice messages from another phone. Please note that you need to set a PIN before you can check your voice messages from another phone.