An interworking solution only requires that Greenfield LTE deploy the IWF and IMS AS, and set up the connections between their MME and IWF for the IMSI attach procedure. The solution uses an IMS client that can be installed on the roamer's devices. Thus once the roamer is attached to the LTE network together with IMSI attach via IWF function, and registered into the IMS system, the roamer can receive voice call and SMS by the original phone number, even though the LTE operator network does not support CSFB.