how come i get calls from strangers saying i called them

what does it mean if i keep getting calls that goes directly to voicemail. its from different people but the same number (everyday). i cant even answer because the phone does not wrong it just goes to voicemail and the voicemail is just random people saying "hello? who is this? helllooooo? *hang up* ". my phone works fine i can call people and get calls and no i did not butt dial them so its something with fizz. my voicemail keeps getting full because of that. i tried blocking the number but i still get the issue. when i try calling back i get a sound as if i was redialling another phone number and it hangs up. please help.