I need to plead ignorance here; I'm not familiar with the jargon and I'm afraid of miscommunication. When I said I had used it overseas, I meant that I had had an eSIM (Airalo, in my case) and that I made local calls to USA using what I understood to be IMS (not connected to hotel or other Wi-Fi but still able to make a call using a physical SIM that had no roaming partner in the area). If there is something more to it, I must ask for a pointer to a web page I can try to understand.

Wikipedia is my go-to reference, but it didn't return anything for Dual SIM Dual Standby. It had Dual SIM Standby (one SIM active at a time) and Dual SIM Dual Active (both SIMs receiving calls), but neither seemed to be what we're discussing. It also didn't have IMS. Usually it serves me well, but not in this case.

Based on having used a Consumer Cellular SIM with no overseas service to make a phone call to USA with no Wi-Fi connection (but with an Airalo SIM with local non-voice service), I assumed I could accomplish the same thing with US Mobile.