Voicemail is related with your network provider, so you should ask them how you can check your voicemail, usually you need to dial a specific number. You can always check if they offer Visual Voicemail with them and ask them to enable it. You can learn more about this feature here - https://support.apple.com/en-us/HT201436 and you can check here if the carrier in your country offers it - https://support.apple.com/en-gb/HT204040.