I've been getting calls from this number the past few days, often late at night. I answered a couple of times and just heard mumbling. I asked who the hell they were and got no answer. Absolutely no idea what's going on here.

i have also received phone calls just of someone mumbling/whispering definitely a mans voice

Yeah same, weird mumbley f*ck head

I keep getting calls and I call back and it either gets rejected and sent to voice mail or they answer and I just hear vibrating in the background.

block this number

Called me a few times randomly. I said hello and no response. Blocked the number

Exactly the same as above 5 times in a row