Random Voicemails, but no missed calls?

Has anyone else experienced this issue? I will leave my phone in sleep mode for a few hours at a time, and then I will hear that I have either a text message or a voice mail from three hours prior. I notice NO missed calls in my log, just voice mails that have suddenly showed up hours after they were left. I've tried to restore, and it will work normally for a day or so and then suddenly I have these mysterious voice mails. I also thought it was because the signal in my house is a little bad, so I did a little experiment one day and stayed in a problem-free signal area and I still experienced this issue - it's very consistent. Is this something I need to take to Apple, or is there something else that I can do to save the hassle and handle it myself? Could it also be that the signal in my area is bad, and I'm just misjudging it or is there anyone else experiencing this? It's quite frustrating, and I hope that it's a simple fix that I have overlooked! If any one can offer me some help, I would greatly appreciate it - it will ease my technology confused mind! 🙂