If it's isolated occurrences, it's the service. I often forward my office phone to my cell phone when I'm going to be out, and I get terrible service in my office. The land line will "ring" a few times before my cell phone picks up service, if it ever does - otherwise it will ring and go to voicemail. (FWIW, for the forward to activate, the recipient phone must pick up, whether by me or my voicemail). Sounds like someone is just trying to call you when you don't have service.
However, if it's happening every call, it could be your phone.