There are two main ways by which SMS can be delivered over LTE system.
• SMS Over SGs: Here LTE UE device sends and retrieves CS(Circuit Switched) based SMS messages.
• SMS Over IP: Here LTE UE device sends and retrieves SIP PS(Packet Switched) based SMS messages.
SMS over SGs transfer the messages between MSC and MME and transports the messages over LTE system using SGs interface. In this type the LTE UE does not fall back to legacy cellular systems such as GSM or CDMA. Existing roaming conditions will be reused in this type of SMS over LTE.
In SMS over IP mode, SIP based SMSs are carried via IMS core element. In this type of SMS over LTE, SMS is to be transmitted is encapsulated in a SIP message in IP-SM-GW entity. This type need LTE UE having both IMS and SIP protocol stack as well as IP Short message gateway(IP-SM-GW), IMS core as well as HLR/HSS(home subscriber server) which supports SMS over IP with the help of home routing.
Hope this helps. Tried to find the Call flow but could not locate :(