No voicemail notification on iphone

[flo]

I installed the Dell Voice app on my iPhone 4S. I tried to call my Dell Voice number. On my iPhone, I saw the incoming call notifications on my iPhone. I did not pick up the call and left a voicemail on my Dell Voice account. After waiting for couple hours, I still have not seen a notification for the voicemail message I left. Is it a bug? Or is there something I need to configure to receive voicemail notification? I have confirmed that the Dell Voice Notification settings on my iPhone is turned on. Thanks in advance.