I do not see a charge for VM unless I actually call my VM to listen to the message? If I were to access the VM from the dialer, long press one or tap the notification, I am charged per minute.
If I access the VM from the native Sprint VM app over WiFi I incur no charge.
I'm having the same issue, I didn't check the VM via the phone's dialer. People leave voicemails and I'm being charged for them. So if I get several robocall voicemails a day, I have to pay for all of them even if I decline them. Very disappointing..