After upgrading from an iPhone X to an 11 Pro Max, I can finally stop using a second device for my second SIM card.
As a frequent traveler to Serbia, my home country, I often need to use my Serbian SIM card when I'm there. I wanted to avoid constantly switching SIM cards in my iPhone, so I got myself an Android phone specifically for my secondary SIM card. But that meant that I couldn't get iMessage on my Serbian phone number, plus I had to carry different chargers and two devices on my trips.
Now with eSIM support (since iPhone XS/XR) I was able to switch my Austrian Telering SIM card to an eSIM profile and use my Serbian VIP Mobile SIM card in the same iPhone.
When I went to the T-Mobile shop (Telering is their brand) to get my SIM converted to an eSIM profile, first they told me that Telering doesn't support eSIMs, only the more expensive T-Mobile does. I was expecting an answer like that, so I showed the agent their own website describing eSIM support. He wasn't sure how to get it done, so he asked one of his colleagues, who said that it's a paid subscription service – which was also wrong.
As the Telering FAQ says, eSIM profile for your main plan is completely free. The only additional subscription cost would incur if I had a separate eSIM profile for an LTE Apple Watch, which Telering doesn't event support yet.
After some fiddling with his computer, the agent was able to set up the eSIM profile on my phone, I thanked him and finished the set up on my own: I inserted my Serbian SIM card and named the two cellular plans appropriately.
I'm not describing this process to be a smartass, but to let you know that you might need to argue with the support agent, since eSIM support seems to be new for them.
I chose my primary plan (Austrian Telering eSIM) to be used for both Cellular Data and as a Default Voice Line. But when I travel to Serbia, I switch the Cellular Data and Default Voice Line to the Serbian plan, and disable Data Roaming. Data Roaming within the EU works great, but since Serbia is not in the EU and my home town is very close to two other countries, I want to avoid accidentaly using data on another network.
Now that my phone has two cellular plans live at the same time, iOS lets me configure and choose which plan I want to use for which call or contact.