When trying to reach your voicemail account, the system requests a number to listen to your messages. Instead of asking for the password, the same welcome message is repeated over and over again.
When accessing the voicemail from an external system, or due to some other network restrictions, you will be prompted to introduce a number before your PIN. However, this number must include the country code, using '00' in place of the '+' symbol. For example, in the case of Great Britain, this should be 0044.
After introducing the correct number, using the country code, the PIN will be requested, and you will be able to listen to your voicemail.