Without EPLMN definition on USIM, is it possible to make VoLTE handover 4G to 4G inter operator via S10 interface?

Without EPLMN defiition on USIM, is it possible to make VoLTE handover 4G to 4G inter operator via S10 interface?

I think no, it will not work.

Likewise SRVCC or CSFB to 2G 3G (ePLMN operator has).

Any expert can confirm?