Can an operator require that SMS always go over IMS and refuse to handle SMS over SG?
Or is SMS over SG a core requirement that always must be available?
Unfortunately the Answer is Yes.you cannot get rid of SMS over SGsAP.The explanation which I can provide you as to why is current SMS is Ideally a CS service.So SMS over SGsAP is is its goto service.SMS over IMS or SMS over sip is a recent development.But you can shutdown the GWMSC once 3G switch off but that can cause adverse effect on the network specially on VAS services like SMS.So When you switching off 3G remember you still cannot shutdown your GW MSC.Not just yet.I have seen some recent 3GPP documentation with some new interfaces beteen AMF and IMS which ideally do the same thing as Sg to get rid of MSC but I am not sure anyone implimented it yet.