The invention concerns a device (D) dedicated to controlling access of a terminal (UE), connected to a CS domain of a mobile network comprising a MSC center (VMSC) running CS communications, to a IMS network services of which its user is a subscriber. Said device (D) comprises: i) a GMSC module managing setup of communications to a terminal (UE) connected to the CS domain, ii) a MGCF module converting messages from the CS domain in accordance with a ISUP protocol into messages in accordance with a SIP protocol addressed to a S-CSCF module and inversely, iii) an I-CSCF initiating registration of terminals connected to the CS domain, with a S-CSCF module, and iv) a P-CSCF emulator (P1) and a user agent (UA1) responsible a) for co-operating together on behalf of the terminal (UE) to instruct an I-CSCF module to initiate the registration of a terminal with a S-CSCF module of the IMS network of which its user is a subscriber, b) when the user attempts via the terminal (UE) to set up a call, for sending said call, via the MGCF module, to the S-CSCF module with which it is registered, and c) upon reception of a request for setting up communication to the terminal, from the S-CSCF module of the TMS network of which its user is a subscriber, for instructing the GMSC module, via the MGCF module, to manage said setting up on the CS domain side.