I have an iPhone with physical sim for my U.S. # and an esim with a French phone #. When I arrive in France next week, I will switch the phone to prioritize the French phone #. My question is IF someone sends a text/iMessage to my U.S. #, will I be able to access the text while my phone is using the French #?
I'm not 100% certain but I believe if the sender is using an Iphone (or any other Apple product) the text will go through on either SIM. This is because the message is going through Apple's network via your Apple Account. If the sender is on Android it will only go to the US # because those texts are routed via the cellular networks.
In short- yes.
Your phone may as igpf you want to keep your US number for AppleID/iMessage. You do.
Messages from iPhone users at home should come in as blue—those are sent and received by the data network (in this case, your French esim.)
SMS messages from the US will come in a s green. Caution that replying to these could trigger international roaming costs.