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

4G LTE
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1

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?