When does Black Butler take place? In chapter 3 or 4 of Black Butler, Sebastian is using a mobile phone when talking to those guys who are holding Ciel captive. 

Is this a mistake by the author or something else? Isn't Black Butler in the 1800s?
In the manga chapter 27 page 41, it was shown that Ciel Phantomhive was born in 1875. Googling with the key phrase "the first cell phone" tells us that the first mobile-phone was created in 1973 by Motorola, which is 100 years after Ciel was born.

Since Ciel was 13 (12 during the first 14 chapters) in the story, mobile phones should not have existed. Thus, like lentinant said in his comment, it is likely that the story takes place in an alternate reality.