I would think they would go to voice mail on your old number if you have the eSIM for the new number on. Not sure how having more than one eSIM on your iPhone works so I am just guessing though.
If both SIMs are active at the same time, you will receive and be able to place calls using either number. When you dial out there will be an option to select which number you wish to call from regardless which is set as primary. The only thing that happens with the “secondary” line is its data connection won’t be in use while it’s set as secondary unless you select it directly in Cellular Settings. See: https://support.apple.com/en-us/HT209044 for more information. Please mark this as the solution if you find I’ve answered your question. Thank you.