Call forwarding

I couldn’t understand why I wasn’t getting any voicemails so my son found a code to put in to see if someone had hijacked my voicemail an sure enough the was being forwarded to an unknown number I was told to go into my settings an turn off call forwarding when I did my call forwarding wasn’t even turned on. I can’t get it to where I get my voicemails bc I don’t know who did or how they could forwarded my messages to their number without call forwarding being off. How can I get my voicemails to my phone an can I find out who did it. If so how