Both are OCN dramas that involve suspense and crime with strong ambitious main characters trying to solve the mysteries they come across. If you are looking for a drama that has the same type of intense dark storyline just like in The Guest, Voice is definitely a good pick to watch. Not to mention, both dramas are directed by the same director, Kim Hong Seon, who did a fantastic job with Voice, and his talent shows here in The Guest too. Kim Jae Wook is also in both dramas so make sure to check Voice out if you want to see more of his charismatic acting!