Hi prota and welcome!
You may have heard conflicting info because citations don't count as much in terms of moving ranking up or down as they once did, but they are still important.
You asked about adding it to A citation as opposed to all your citations. So if I take your question literally, just one citation should not throw things off.
But the other problem is if lots of citations have the 800 # it could cause Google to create a duplicate listing for your business and THAT will usually hurt your ranking, plus can create other problems potentially like stealing some of your reviews AND it's time intensive to get duplicate problems resolved.
So the big question is - IF you are a local business (which you should be in order to even qualify to have a G+ Local page) then why do you even need to show an 800#? You will likely only rank in the city you are located in and to most potential customers, it would be a local call.
The other point I'll make is few customers will search a directory to find you compared to the # that could search Google to find you. So I would think pleasing Google and keeping that listing healthy and ranking would outweigh any directory listing???
Note: I'm talking blind with all the comments above, because all this depends partly on what industry and what directory we are talking about. For instance if a hotel, could be more important to list 800#.