A method of establishing a media session between a first mobile station and a second mobile station, the method including receiving, at an MQTT messaging service and from the first mobile station, a first MQTT subscription request for a first topic name used to indicate the second mobile station agrees to establish a media session with a mobile station; receiving, at the MQTT messaging service and from the first mobile station, a first MQTT application message having a second topic name used to indicate a mobile station requests establishing a media session with the second mobile station; issuing, in response to receiving the first MQTT application message, an indication to the second mobile station that a mobile station requests establishing a media session with the second mobile station; receiving, at the MQTT messaging service and from the second mobile station, a second MQTT application message having the first topic name; forwarding the second MQTT application message to the first mobile station; receiving a SIP INVITE message issued by a third mobile station, the third mobile station being one of the first mobile station or the second mobile station; forwarding the SIP INVITE message to a fourth mobile station, the fourth mobile station being one of the first mobile station or the second mobile station and not the third mobile station; receiving a SIP OK message issued by the fourth mobile station; and forwarding the SIP OK message to the third mobile station.