This is a unique feature specific to Dual SIM enabled phones. When no service is detected on your primary line (e.g. T-Mobile), your phone will switch to WiFi Calling using the data from the secondary line (e.g. eSIM data provider such as Airalo or local SIM card if present). It will show as "<Carrier> using Cellular Data" on your iPhone when this is working correctly.

You can do so by disabling international roaming (or more broadly, cellular data) on the primary line and setting your phone to use the secondary line for data.

If you're curious and want to read more, the specific name of this is IP Multimedia Subsystems (IMS).