Method and system for initiating a mobile voice call

Method for initiating a mobile voice call from a caller user equipment (UE) to a callee UE, comprising the following steps:

• a) providing to the caller UE a software function arranged to execute on or from the caller UE;
• b) the caller, using said software function, selecting a callee phone number compliant with the phone number recommendation E.164;
• c) the caller UE communicating, to a session initiation protocol (SIP) server, the callee phone number;
• d) the SIP server (41) determining whether or not the callee phone number is associated with a callee UE for voice over internet protocol (VOIP);
• e) if so, the SIP server identifying the callee UE using a SIP address the “user” field of which comprises the callee phone number, initiating the call between the caller UE and the callee UE as a VOIP call;
• and
• f) if not, the SIP server initiating the call between the caller UE and the callee UE as a circuit switch (CS) call using the callee phone number and using a network switching function of a first network to which the callee UE subscribes.

System for initiating a mobile voice call from a caller user equipment (UE) to a callee UE, which callee UE has a subscriber identity module (SIM) card associated with a callee phone number, which callee phone number is a E.164 compliant Mobile Station International Subscriber Directory Number (MSISDN) of the SIM card, which system comprises:

• a software function, arranged to execute on or from the callee UE;
• and
• a voice over internet protocol (VOIP) service arranged to accept circuit switch (CS) voice calls forwarded to a forwarding phone number from a first network to which the callee UE subscribes to the VOIP service,
• wherein the software function is arranged to, when executing, automatically detect the callee phone number from said SIM card, and thereafter automatically to provide the VOIP service with the callee phone number and/or a receiving address comprising the callee phone number,
• wherein the VOIP service is arranged to identify the callee UE based upon the forwarding phone number, and to, as a result of this identification, initiate the call between the caller UE and the said software function, using the receiving address, and
• wherein the receiving address is a session initiation protocol (SIP) address in which the “user” field comprises the callee phone number.