USIM - Universal Subscriber Identity Module
ISIM - IP Multimedia Services Identity Module
ISIM and USIM are applications which run in your simcard. Both applications are responsible (among other things) to perform security procedures such as authenticate the network. They also carry your identity and security keys used to encrypt the information transmitted over-the-air.
ISIM carry a lot of information(files) used in SIP protocols such as your IP Multimedia Private Identity (IMPI), domain, IP Multimedia Public Identity (IMPU) and cypher keys (used to encryp information. So, this application is used for SIP/IMS procedures - consequently, VoLTE calls.
USIM does not have that kind of information. It has instead IMSI, SPN, Cyphering and Authentication keys used during registration in UMTS/LTE networks.
In a simple way, ISIM is used during SIP/IMS procedures as long as USIM is used during NAS/RRC/MM/GMM procedures.
Even then, I believe that you don't need a ISIM to register in IMS (and make a VoLTE call). 3GPP defines standards which make possible to create a IMPI/IMPU using keys from your USIM.
Even then, as I observed, most part of network carriers replace their old SIMCard which has only the USIM application by a new SIMCard which has both applications (since they can co-exist in same SIMCard).