iOS (the iPhone operating system) uses a system similar to Trash on a Mac, in that deleting a voicemail will remove it from view but not necessarily remove the file completely. If your iPhone is still picking up that file, it will show as not being able to be deleted.
Voicemails have been around in one form or another since the beginning telephone communications. This feature still manages to make its way on every smartphone, despite the rate in which technology is evolving. The reason for this is usefulness – you won’t miss any unanswered calls even if your phone is off. The voice messages from your callers will be transferred to your voicemail box and you’ll be able to listen to them at a later date.

You can also set your Galaxy A series smartphone to record all the calls (or selectively record calls to and from select numbers) automatically without the need to hit the record button. If you frequently record calls, this feature is quite useful, and here’s how you can do it:
Some networks will provide a special number for you to ring, although with O2 and some others you just have to dial your own mobile phone number and then hit the * key to enter the voicemail system. Again, you’ll need to make sure that you’ve set up a secure PIN for your voicemail to make sure that none of Rupert Murdoch’s minions can hack into your voicemail from the comfort of their desk.