How can I prioritize and control data usage with eSIM on iPhone 15 while maintaining separate business and personal lines?

I have an iPhone 15 latest update 18.3 as of February 2025.

My primary telephone has an unlimited data package, so it seemed like a slam dunk to just add a voice only business line with an eSIM and within minutes I would be up and running. I got the eSIM and tested it for a few days at home under a trial, and then purchased plan with a full commitment and have since changed all my business cards in my business contacts to this new number.

the instant someone calls me on that business number that I cannot use any data if I want to check or reference something online, unless I am also on Wi-Fi. WiFi is not an option, I’m usually traveling, and I cannot be using much data on the business line because I’m going to pay for two data packages, that’s unacceptable.

This means my choices are

leave cellular data switching off and not be able to use any Internet at all while I’m on the business line, which is unacceptable

2.Manually go to the settings and allow cellular data switching every time. I’m on the phone and simultaneously would like to use the Internet and then go back to the settings and turn it back off when I’m done with the call, this is beyond, far beyond totally unacceptable

3.Allow cellular data switching and do what I gotta do and just pay the business line bill when it comes, no matter how high it is, even though I’ve already paid for unlimited through my primary carrier, which is unacceptable

4.Carry two phones that way 1 would basically just be for calling and texting and my iPhone would be back to its old self and I wouldn’t even have to worry about this, which is totally unacceptable

5.get rid of the business line, and throw away all my business cards that I just ordered and get new ones with my personal line and spread my personal line that I’ve had for almost 30 years all over the Internet so that I get overridden by spam and have to cancel that number and get a new one, which isn’t gonna happen because that is totally unacceptable

Can someone please explain to me what I am doing wrong, because as detailed and smart and thought through as everything is on every Apple device, I have a super hard time believing that there is not a way to prioritize my data so that I only use out of the unlimited plan, except for I would be OK with the occasional tiny bit of data that I might need if I’m referencing something online while simultaneously talking on the business line. Most of my calls would likely continue on the personal line so the amount of data that I would need while talking on the business line would be a price I’m willing to pay. But what I’m not willing to do is let the phone quietly without me knowing burn through $50 of data on my business line while I have another $50 in data that’s unlimited just sitting there untouched.

From everything I’ve read when you have cellular data, switching enabled, you do not have control, or even knowledge of which one is being used at any given time, which means you have no clue what your cellphone bill will be.

This totally defeats the point of having two lines on an iPhone with eSIM

And while I’m asking these questions, I would like to reference back to the beginning of this and ask, why is there not a way to set business hours for either of my lines to direct call straight to voicemail? My parents had a phone. The automatically sent all calls to voicemail after a certain time back in the 1990s, so like why is that not an option, I cannot be the only one that has an iPhone and is also running a small business

[Re-Titled by Moderator]