You're right. I remember the anime changed it to a field telephone (look it up) to make it work, but the original manga had a 90's style flip cell phone in chapter 3.
Canonically though, this story takes place in a technologically advanced Victorian era. So, a lot of things that weren't around in that time period are in this manga. For example, the German Panzer tank wasn't a thing until WWII, yet its working prototype is available almost 50 years earlier. But the cell phone bit is certainly very silly.
I could be wrong and I don’t remember which chapter specifically, but it was early in the Curry Arc when Ciel and Soma has just gotten to know each other, where there was one panel in which Sebastian tells Ciel he was able to distract Soma. The “distraction” was that Soma was watching TV so yeah...early in the manga kinda threw more historical accuracy out of the window than the recent arcs lol.
the first volume has always bugged me it seems to take place in the late 1990s to early 2000's time but the rest take part in the late 1800s too early 1900's I love the rest of the series don't get me wrong but I have mixed feelings about the first volume