Recording voicemails?

Okay because I don't really understand if Illinois is a one party or 2 party consent state (I've read basically all the same things online and I really can't figre out which one it truly is) I'm assuming recording phone calls is illegal and cannot be used as proof of harassment, emotional abuse, etc. in court. BUT can you record voicemails and use those? I mean the person rightfully knows they are being recorded if they leave you a voicemail right? My ex continues call "drunk dial" (for lack of a better phrase) me and I have been logging every call (because I do not answer them they are all in my missed call list, if I answer them, even long enough for them to show up on my phone bill this eggs him on to call me more, I've found that out. So I write them all down) I am also logging every time he backs out of visitation (and most times he leaves the lame reason or blames me for something as to why he's not taking them on my voicemail) basically I'm logging everything because I am planning on going back to court in the fall. I just want to make sure I have enough evidence to back up my claims and I know that will give me time to do so. So my question is, is recording voicemails onto a tape recorder usable legally?