Abstract: A method for establishing bi-directional and session-based communication between IMS and GSM subscribers with the purpose of sending text information by using the SIP protocol and USSD services. The method defines an IP-USSD gateway that converts messages between the SIP and USSD protocols before relaying them. The communication session can be initiated by either party; the GSM subscriber or the IMS subscriber. A message, which an SIP client has sent, is thereby converted into a USSD message and conveyed to the GSM subscriber. Thus, the GSM subscriber can read the USSD message sent to the mobile phone, and in addition can see the name and surname of the sender and write an answer. Similarly, the SIP subscriber can read the message received and write an answer.