Your cell phone doesn't have to be registered with Fidelity. If your present or former employer has it, then your 401k or pension fund admin, if Fidelity has it. You employer forwards all contact info to Fidelity.
I received the same repeated phone calls to my cell phone from this number. My cell phone is not registered with Fidelity. I did not answer and they refuse to leave an answer
I've received calls from Fidelity in the past to discuss my 401k. It's an annoying sales call, but reputable. However, the Fidelity calls always included a message and a respectable caller id. I wouldn't trust anyone with UNAVAILABLE as a caller id.