Method to establish a secure voice communication session between two user equipments (UE1, UE2) with the help of a dedicated Network Application Functions and of at least one Bootstrapping Server Function (BSF1, BSF2), comprising the steps of: - for a first user equipment, sending a request (REQ(ID1,ID2)) of communication with a second user equipment and a request for security association (REQ(ID1,ID2),SEC) to a dedicated Network Application Function, - for the first user equipment, proceeding to a challenge procedure (CH1) comprising: - for the first user equipment, establishing a link with a first Bootstrapping Server Function BSF1, - for the BSF1, transmitting a challenge to the first user equipment, - for the first user equipment, responding to the challenge transmitted by the BSF1, - for the BSF1, verifying the challenge response, - for the NAF, retrieving bootstrapping service derived NAF keys (Ks_(ext/int)_NAF1) from the BSF1, - for the second user equipment, receiving a request (REQ(ID1,ID2)) for communication with the first user equipment, - for the second user equipment, sending a request for security association (REQ(ID1,ID2),SEC) to the dedicated Network Application Function, - for the second user equipment, proceeding to a challenge procedure (CH2) comprising: - for the second user equipment, establishing a link with a second Bootstrapping Server Function, - for the BSF2, transmitting a challenge to the second user equipment, - for the second user equipment, responding to a challenge transmitted by the BSF2, - for the BSF2, verifying the challenge response, - for the NAF, retrieving bootstrapping service derived external and internal NAF keys (Ks_(ext/int)_NAF2) from the BSF2, the method further comprising the steps of: - calculating a session key (Ks_SV) from bootstrapping service derived external or internal NAF keys of the first and the second user equipments (Ks_(ext/int)_NAF1 and Ks_(ext/int)_NAF2) and - establishing a secured voice communication using the calculated session key (Ks_SV). - for a first user equipment, sending a request (REQ(ID1,ID2)) of communication with a second user equipment and a request for security association (REQ(ID1,ID2),SEC) to a dedicated Network Application Function, - for the first user equipment, proceeding to a challenge procedure (CH1) comprising: - for the first user equipment, establishing a link with a first Bootstrapping Server Function BSF1, - for the BSF1, transmitting a challenge to the first user equipment, - for the first user equipment, responding to the challenge transmitted by the BSF1, - for the BSF1, verifying the challenge response, - for the first user equipment, establishing a link with a first Bootstrapping Server Function BSF1, - for the BSF1, transmitting a challenge to the first user equipment, - for the first user equipment, responding to the challenge transmitted by the BSF1, - for the BSF1, verifying the challenge response, - for the NAF, retrieving bootstrapping service derived NAF keys (Ks_(ext/int)_NAF1) from the BSF1, - for the second user equipment, receiving a request (REQ(ID1,ID2)) for communication with the first user equipment, - for the second user equipment, sending a request for security association (REQ(ID1,ID2),SEC) to the dedicated Network Application Function, - for the second user equipment, proceeding to a challenge procedure (CH2) comprising: - for the second user equipment, establishing a link with a second Bootstrapping Server Function, - for the BSF2, transmitting a challenge to the second user equipment, - for the second user equipment, responding to a challenge transmitted by the BSF2, - for the BSF2, verifying the challenge response, - for the second user equipment, establishing a link with a second Bootstrapping Server Function, - for the BSF2, transmitting a challenge to the second user equipment, - for the second user equipment, responding to a challenge transmitted by the BSF2, - for the BSF2, verifying the challenge response, - for the NAF, retrieving bootstrapping service derived external and internal NAF keys (Ks_(ext/int)_NAF2) from the BSF2, - calculating a session key (Ks_SV) from bootstrapping service derived external or internal NAF keys of the first and the second user equipments (Ks_(ext/int)_NAF1 and Ks_(ext/int)_NAF2) and - establishing a secured voice communication using the calculated session key (Ks_SV).