How do I switch between SIM and eSIM on iPhone?

Watch the GIF below to change the Voice line. This has to be done each time you receive a call and want to call back from another line other than through which it was received.

Alternatively, you may change the Voice line to be used under Contacts. And dial using contacts and not using the Recent call list.

While making a fresh call using dial pad.

There is a difference between "Last Used" and "Always Use"

"Last Used" : This is waht the Contact has used for contacting you. You can immediately call back or text them from the same number where the call/text was received

"Always Use" : This is what you want to use while making a call/text. Open the Contacts App and Open a Contact and Tap Call/Message. Now it will use your default number.

There is a difference between "Last Used" and "Always Use"

if you have selected a Line for a Contact, Say Line B as "Always Use"

and If that contact sends you a text using Line A then if you reply to that contact using the received message in the Message App as a continuation of the conversation then the Contact will be assigned as "Last Used"

Always start a conversation from the Contacts App if you want to keep Business and Personal lines separate.

If you start the conversation from the old thread then you cannot change it. Start a new conversation from Contacts App. Here you can change the number as well as the Line. As shown in the GIF below

The pic below gives you step-by-step process. This needs to be done for each contact.

On iPhones, there is currently no feature that allows you to toggle or switch between a physical SIM and an eSIM for the same phone number. This means if you have a single phone number activated as both a physical SIM and an eSIM (for instance, when transferring your number from one to the other), the iPhone doesn't offer a built-in option to alternate or switch back and forth between them on demand.

However, if you have two different phone numbers—one on a physical SIM and the other on an eSIM—you can keep both active simultaneously. The iPhone supports Dual SIM functionality, which means you can use one line for calls and texts, and the other for data or secondary communication needs. In this case, there's no need to toggle between the SIM and eSIM; they function together without conflict, and you can choose which number to use for different actions like messaging, calling, or cellular data.

All iPhone models that support eSIM can store multiple eSIMs and use Dual SIM with two active SIMs at the same time. You can use Dual SIM by ...

Using Dual SIM with an eSIM

You may change the Voice line to be used under Contacts. And while making a call dial using contacts and not using the Recent call list.

There is a difference between "Last Used" and "Always Use"

"Last Used" : This is waht the Contact has used for contacting you. You can immediately call back from the same number where the call was received from the Phone App > Recents Tab

"Always Use" : This is what you want to use while making a call. Open the Contacts App and Open a Contact and Tap Call. Now it will use your default number.

Normally...

Watch the GIF below to change the Voice line. This has to be done each time you receive a call and want to call back from another line other than through which it was received.