LTE: How a LTE UE come to know whether LTE network supports IMS call or not ?

Question

Does LTE network send any specific message to UE so that UE come to know network supports IMS call ?
Or
Does the LTE network broadcast information such as "it does not support IMS call" ?

Answer 1

Sometimes UE gets IP address of P-CSCF as part of attach procedure, it is a kind of notification from network to UE that it has IMS service support. I also observed even after the attach procedure, when UE want to initiates IMS signalling with IMS domain nodes it should have P-CSCF IP address since signalling message flows through P-CSCF, I-CSCF, S-CSCF nodes.

For that purpose, UE does P-CSCF discovery by using DNS queries. As far as I know, network does not broadcast saying "it does not support IMS service". However, network will send or not P-CSCF address to UE, it depends on received PCO "Protocol Configuration Options" from UE in the attach request message.
If UE has requested for primary DNS ip address, secondary DNS ip address etc. PGW includes responses for all the received request.

Answer 2

In attach accept, core network informs UE about IMS voice over PS session in S1 mode supported or not for the UE as below.

EPS network feature support
.... ...1 = IMS VoPS: IMS voice over PS session in S1 mode supported.

If UE is attached other then qci 5 bearer and have above information in attach accept, UE again performs pdn connectivity request for ims apn. UE also sends P-CSCF address request in PCO. Then core network sends erab setup request with qci 5 and also sends p-cscf address.

Now UE can perform IMS registration using p-cscf address and then can make volte call.