UE can be addressed by S-TMSI or IMSI (depending on the scenario).
As specified in 3GPP 24.301, UE identity in Paging message can be either S-TMSI or IMSI.
Paging with IMSI is an abnormal procedure , and it is used for Error Recovery.
The network may initiate paging for EPS services using IMSI with CN domain indicator set to "PS" if the S-TMSI is not available due to a network failure.
(like MME restart, and lost all S-TMSI information, or )
As stated in the Spec, when UE receives a paging with IMSI , it should locally deactivate any EPS bearer and locally detach from EPS, that is UE will turn into EMM-DEREGISTERED state.
After then, UE will initiate Attach Procedure again.