The ringing telephone in the catacombs that Scarlett (Perdita Weeks) answers, is a circa 1940 Stromberg-Carlson desk phone. However, this one was modified with a receiver from an earlier ca. 30's model, and the more modern spiral cord. Rather than a traditional cloth cord. The Stromberg-Carlson was widely used in posh hotels; and can be found in a large number of films, particularly noir, from the '30s through today.
This was the first ever production that secured permission from the French government to film in the catacombs. The film utilizes a set of narrow, winding tunnels of the Paris catacombs, complete with real mint skeletons creepily arranged centuries ago.
In the catacombs all the main characters wear headlamp/camera combinations. The headlamps point upwards, yet the "footage" the are supposed to film is pointed downwards.
When they show the house of Flamel, they show that the house is on rue de Flamel, this is not true. Flamel's house is on 51 rue de Montmorency, rue de Flamel is only named after him.
The "Abandon all hope, ye who enter" inscription is a quote from Dante Alighieri's Divina Commedia ("Lasciate ogne speranza, voi ch'intrate"), and not an "old legend".
It is strongly suggested as the group crawls through the tunnel with an inscription George reads as "Abandon all hope ye who enter here". Which Scarlett then responds with something along the lines "...and they would be made to crawl on their bellies into the darkness of hell". It is after the group emerges from this tunnel that things begin to go particularly wrong.
Q
What is up with the devil worshipers?
A
It is left completely unexplained. One could assume they were satanists.
Q
In the phone scene, who/what was on the other line and what did they say?
A
It is assumed that it is Scarlett's dead father calling from hell (because he killed himself by hanging as he is seen at the beginning of the movie during the bombardment) asking her why wouldn't she talk to him. He is referring to the night he killed himself, after he tried to call his daughter but she wouldn't answer, as she tells him near the end of the movie when she sees him again and apologize before he disappears.