To answer your question, Voicemail received while you are outside the US will incur charges.
Specifically, voicemail deposits and retrievals are charged at the roaming rate of the country you are in. You are charged one minute for delivering the call to your device, even if you do not listen to the voicemail.
Additionally, if your device is off and a call goes to voicemail, you will not be charged. However, once you turn on your device and it registers on international towers, any voicemail received while the device was off will be charged
Aside from this, Do you have other inquires that we can help you with?