Method and system to include a TETRA member (subscriber) in an ongoing PS-LTE group call using SS-LE, by mapping a plurality of specific SS-LE messages is disclosed. The embodiments herein enable TETRA SS-LE service to interwork with PS LTE GCS. The process involves the steps of queuing the TETRA call leg at the PS LTE GCS AS, informing the TETRA SwMI to activate SS-LE for a TETRA member, embedding a SS-LE PDU as a payload in the SIP messaging module, defining and interrogating a SS-LE service on a GCS AS via the authorized user on the TETRA SwMI or on the PS-LTE network. The TETRA member is connected to the ongoing group call through a gateway. The TETRA SwMI works as the TETRA core and the LTE to TETRA Gateway facilitates the process of message mapping and protocol conversion.