On behalf of Bart -
This looks like a tricky question, but the answer to it is very simple.
IMS signaling for LTE network is just dumb payload. Like porn or CNN webpage.
How? Again, quite simple. IMS signaling does have nothing in common with Core Network. Mobile Network is transmitting it from UE to the Internet and back like it's transmitting the CNN webpage or porn. Same way, nothing special. Maybe except the QCI which is given to it, because GGSN/PGW was told by configuration to do so, or by PCRF if the P-CSCF has Rx with PCRF.
As you know, or at least you should, IMS uses SIP protocol to operate. SIP protocol except being a monstrous TEXT(sic!) protocol it does not interact with any Core node/element. It's pushed to its destination in a GTP Tunnel like everything else, where just before the Gi/SGi it's decapsulated and sent out to the Internet.