A method for performing a call setup from a first terminal residing in a Circuit Switched telecommunication network to a second terminal residing in a Packet Switched telecommunication network, the Circuit Switched network and the Packet switched network being communicatively connected to each other by a network gateway entity, and a network entity controlling the network gateway entity, the method comprising the steps of: - receiving (32, 52, 62), at the network entity from a Short Message Service Center, SMSC, a first message, the first message being triggered by the first terminal when sending a Short Message Service, SMS, message to the address of the network entity, which comprises the Calling Party Number, CgPN, of the first terminal and an address of a party to be called at the second terminal, whereby the address of the network entity and the CgPN are in E.164 address format and the address of the party to be called at the second terminal is a Session Initiation Protocol - Universal Resource Indicator, SIP-URI; - storing, by the network entity, the first message in a database (13A, 42); - receiving (34, 55, 65), at the network entity from a Mobile Switching Centre, MSC, a second message, the second message being triggered by the first terminal when sending a call setup request to the address of the network entity, which comprises the CgPN of the first terminal, and whereby the address of the network entity and the CgPN are in E.164 address format; - regarding, by the network entity, the second message as a request of the first terminal to call a party at a terminal residing in the Packet Switched network, the party being represented by the SIP-URI in the first message; - querying, by the network entity, the database (13A, 42) for the SIP-URI of the party to be called; - selecting, by the network entity, from the database (13A, 42) the stored message comprising a CgPN which corresponds to the CgPN received with the second message; - retrieving (55A, 55B; 65A, 65B), by the network entity, the address of the party to be called in SIP-URI format from the selected stored message, and resolving the address of the party to be called into an IP address of the second terminal; - setting up the call (36A, 36B; 57A, 57B; 67A, 67B) between the first terminal and the second terminal identified by the resolved address of the second terminal by the network entity sending (35B, 56B, 66B) a SIP INVITE message with the SIP-URI to this IP address, and - connecting the call (37, 58, 68) via the network gateway entity. - receiving (32, 52, 62), at the network entity from a Short Message Service Center, SMSC, a first message, the first message being triggered by the first terminal when sending a Short Message Service, SMS, message to the address of the network entity, which comprises the Calling Party Number, CgPN, of the first terminal and an address of a party to be called at the second terminal, whereby the address of the network entity and the CgPN are in E.164 address format and the address of the party to be called at the second terminal is a Session Initiation Protocol - Universal Resource Indicator, SIP-URI; - storing, by the network entity, the first message in a database (13A, 42); - receiving (34, 55, 65), at the network entity from a Mobile Switching Centre, MSC, a second message, the second message being triggered by the first terminal when sending a call setup request to the address of the network entity, which comprises the CgPN of the first terminal, and whereby the address of the network entity and the CgPN are in E.164 address format; - regarding, by the network entity, the second message as a request of the first terminal to call a party at a terminal residing in the Packet Switched network, the party being represented by the SIP-URI in the first message; - querying, by the network entity, the database (13A, 42) for the SIP-URI of the party to be called; - selecting, by the network entity, from the database (13A, 42) the stored message comprising a CgPN which corresponds to the CgPN received with the second message; - retrieving (55A, 55B; 65A, 65B), by the network entity, the address of the party to be called in SIP-URI format from the selected stored message, and resolving the address of the party to be called into an IP address of the second terminal; - setting up the call (36A, 36B; 57A, 57B; 67A, 67B) between the first terminal and the second terminal identified by the resolved address of the second terminal by the network entity sending (35B, 56B, 66B) a SIP INVITE message with the SIP-URI to this IP address, and - connecting the call (37, 58, 68) via the network gateway entity.