We develop a robust multiuser detector for a Frequency Division Multiplexing (FDM) system where each user corresponds to a Bluetooth transmitter employing a binary Gaussian Frequency-shift keying modulation (GFSK) characterized by a significant modulation index uncertainty, and sent over a channel affected by phase noise. The proposed receiver is designed by adopting a simplified representation of a binary CPM signal with the principal component of its Laurent decomposition and is obtained by using the framework based on factor graphs and the sum-product algorithm.