In a system to control a VoIP service, a user terminal performs a first registration procedure of a CSCF through a first P-CSCF using a first mobile IP allocated from a wireless LAN, monitors a signal intensity of the wireless LAN while receiving the VoIP service over the wireless LAN, and performs a second registration procedure through a second P-CSCF using a second mobile IP allocated from a wireless packet network upon booting if the signal intensity of the wireless LAN becomes weaker than a reference value. The CSCF sends a VoIP call to the second P-CSCF if the user terminal performs the second registration procedure and the user terminal then receives an incoming VoIP call.