Hello AdamG72,
Thanks for reaching out to Apple Support Communities. We see you have some questions about using Dual SIM, and we'd like to help.
When a call is received you should see a letter in a box below the name of the person or number that is calling. This icon corresponds with the first letter of the label you assigned to the phone number receiving the call. Each of the phone lines would've been assigned a label, for instance "Primary" and "Secondary." If the call is received on the "Primary" line you should see a "P" icon. Likewise you would see an "S" icon for the "Secondary" line. You can change the labels that are currently associated with the lines if you choose to do so.
The following link can help with how to use Dual SIM: Using Dual SIM with an eSIM - Apple Support
Regards.