Method for initiating a mobile voice call from a caller User Equipment, UE to a callee UE, having 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, the caller UE is configured to receive and initiate a Circuit Switch, CS, voice call, a Mobile Switching Center, MSC, is configured to forward the CS call to a Session Border Controller, SBC, the SBC is connected to the callee UE, the callee UE is configured to receive and initiate a Packet Switch, PS, voice call, the method comprises the following steps, performed ahead of the said call initiating: a) providing a software function, the software function is arranged to be executed by the callee UE when the software function is installed on the callee UE, the software function is arranged to be executed on a server communicating with the callee UE when the software function is installed on said server communicating with the callee UE, which software function is associated with a receiving address, which receiving address comprises the said callee phone number, wherein the software function comprises a Session Initiation Protocol, SIP, client which is arranged to connect to a SIP server through SIP signalling over Internet Protocol, IP wherein the software function is arranged with functionality for setting up voice calls incoming and outgoing based on SIP based Voice over IP, VOIP, and wherein the receiving address is assigned to the SIP client part of the software function as SIP address for the SIP client, which receiving address is registered and configured at the SIP server; and b) configuring an unconditional call forwarding function in a first network to which the callee UE subscribes, which call forwarding function is activated for the said callee phone number and configured for forwarding CS voice calls to a forwarding phone number associated with a Voice over Internet Protocol, VOIP, service, and wherein the method further comprises the following steps, performed in connection to the said call initiating: c) the caller UE calling the callee phone number; d) the MSC of the first network automatically forwarding the call control to the SIP server of said VOIP service using said forwarding phone number, by routing the call from the MSC to the SBC, which then forwards the call to the SIP server, the VOIP service is a forwarding destination; e) the VOIP service identifying the callee UE based upon the forwarding phone number; and f) the VOIP service initiating the call between the caller UE and the said software function, using the receiving address as the SIP address used for connecting to the SIP client part of the said software function. a) providing a software function, the software function is arranged to be executed by the callee UE when the software function is installed on the callee UE, the software function is arranged to be executed on a server communicating with the callee UE when the software function is installed on said server communicating with the callee UE, which software function is associated with a receiving address, which receiving address comprises the said callee phone number, wherein the software function comprises a Session Initiation Protocol, SIP, client which is arranged to connect to a SIP server through SIP signalling over Internet Protocol, IP wherein the software function is arranged with functionality for setting up voice calls incoming and outgoing based on SIP based Voice over IP, VOIP, and wherein the receiving address is assigned to the SIP client part of the software function as SIP address for the SIP client, which receiving address is registered and configured at the SIP server; and b) configuring an unconditional call forwarding function in a first network to which the callee UE subscribes, which call forwarding function is activated for the said callee phone number and configured for forwarding CS voice calls to a forwarding phone number associated with a Voice over Internet Protocol, VOIP, service, and wherein the method further comprises the following steps, performed in connection to the said call initiating: c) the caller UE calling the callee phone number; d) the MSC of the first network automatically forwarding the call control to the SIP server of said VOIP service using said forwarding phone number, by routing the call from the MSC to the SBC, which then forwards the call to the SIP server, the VOIP service is a forwarding destination; e) the VOIP service identifying the callee UE based upon the forwarding phone number; and f) the VOIP service initiating the call between the caller UE and the said software function, using the receiving address as the SIP address used for connecting to the SIP client part of the said software function.