This mysterious phone call is to Langdon's literary agent, Jonas Faulkman.
Ah ha. That's what Langdon figured out. He realized his agent must have sent an advance copy of his Holy Grail manuscript to Saunière. And he proposed they should meet. Well, that answers that question.
Teabing finds the whole thing hilarious.
Sophie thinks it could be trouble, because it could implicate Langdon in Sauniére's death, rather than clear his name.
They arrive at the plane hangar, and bribe the pilot (using money and the quite-convincing gun) to take Langdon, Sophie, and Silas in addition to Teabing and Rémy.