dual sim iphone: wifi only for one number when traveling?

Exactly I don't want to connect to cellular data with that number! I'm not asking to do that. I'm asking to do what I can do on any iPhone (other than this dual SIM). To use your example, I can physically turn off cellular connectively in settings on my old iPhone x and still receive iMessages and FaceTime to that cellular number even though the phone itself is offline. That's all I want to do on the dual sim. AT&T does allow it--they have for years.

Thank you so much for the response! That is indeed what I"m trying to do. But unfortunately when I follow your instructions above, I'm not seeing a "voice and data" line to disable! (See photo below) Underneath my number it just says "enable LTE."

When I turned the phone line back on, LTE was enabled so I quickly toggled it off, but I don't think that will help me to limit voice/data to just wifi. Also, under Network Selection there isn't "wifi" to choose. It just allows me to choose automatically or pick a specific carrier.

I tried turning "on" wifi calling after I took this screenshot and it didn't change anything.

What did I miss?

Thanks, Alex. Turning it completely off was what I was trying to avoid. I'd been doing that, but it switched Facetime/imessage to my UK number which prevented me from getting any of my messages on my US number while I'm in the UK. That seems strange that those type of controls would be carrier dependent. I would think that turning off data and not turning off wifi is something basic enough that apple would put in that capability for any user. I can't be the only one who is trying to avoid horrible international rates through a US provider--especially AT&T which has to be one of the biggest. Sigh. Maybe that will be in a iOS future update.

But I've been able to do this for years. See my original post where I had two phones--my US phone that was in airplane mode with wifi turned on (what i'm trying to do on the dual sim phone) that I was able to continue to receive my iMessage and FaceTime messages on my US number, and my UK phone, which was separate. I'm trying to recreate that experience with the dual sim iPhone.

The email address associated with my apple id would certainly work for family members, but it wouldn't work for businesses that use your number to contact you, i.e. credit card companies. I actually missed a fraud text because of this issue. When I switched the number back on momentarily yesterday in my hotel room (hopefully not incurred the $10 att charge), the message came through.

I think I'm making what I want to do sound more complicated than it is. All I want to do is be able to turn off "cellular" on a sim and still be able to receive iMessages/Facetime on that number on the dual sim phone. Right now as soon as cellular is turned off for one number the other number is *automatically* enabled for iMessage and FaceTime--this is an apple functionally not an AT&T functionality. I don't mind having my AT&T phone completely "off," but I still want that number to work with wifi for imessage/facetime. What I'm trying to do is no more nefarious than turning wifi on when a phone is in airplane mode or checking iMessage/facetime on a computer.

It has been allowed for years with every iPhone I've had back until at least the 4 or 5. If your phone is in airplane mode (i.e. everything cellular/data is "off") you can still enable wifi and receive imessages and FaceTime on that phone. If you have an iPhone you can try it with your phone right now to see what I'm talking about by putting it in airplane mode and turning wifi on.

It's the same thing as receiving iMessages or FaceTime over a computer or wifi only iPad (i.e. anything without data) with that same AT&T phone number. AT&T is completely out of the equation on those devices.

Basically all I want to do is turn off cellular on one SIM and still enable wifi on that line. I expected to have some kind of toggle option when I turned off cellular, i.e. "Would you like to enable wifi on this line to use for iMessage and Factime" or something like that. I'd hoped it was something I'd missed, but it sounds like the functionality to do that is not there.

I wouldn't even mind if I could just turn off cellular on that line and still be able to receive iMessages/Facetime on other "i" devices like my MacBook pro and/or iPad with my US number. But the dual sim phone automatically switches the phone number to the other number--in this case my UK number--for iMessages and FT as soon as my US number is turned off on the new phone, basically cutting me off from messages to that number while I"m in the UK.

sounds like a trip to the genius bar might be in my future when I get home. Thanks to everyone for the responses--much appreciated.