Using data sim overseas

Hello cartwebb,

This page can assist with getting Wi-Fi Calling set up for the iPhone prior the traveling: Make calls using Wi-Fi on iPhone - Apple Support

After that, this link will have some details that can provide details regarding making calls via Wi-Fi Calling when Dual SIM is in use: Using Dual SIM with an eSIM - Apple Support

Make and receive calls

You can make and receive phone calls with either phone number.

When you're on a call, if the carrier for your other phone number supports Wi-Fi calling, you can answer incoming calls on your other number. When you're on a call using a line that isn't your designated line for cellular data, you need to turn on Allow Cellular Data Switching to receive calls from your other line. If you ignore the call and you have voicemail set up with your carrier, you’ll get a missed-call notification and the call will go to voicemail. Check with your carrier for Wi-Fi calling availability and find out whether additional fees or data usage applies from your data provider.

If you're on a call and your other line shows No Service, either your carrier doesn't support Wi-Fi calling or you don't have Wi-Fi calling turned on.3 It could also mean Allow Cellular Data Switching is not turned on. When you're on a call, an incoming call on your other phone number will go to voicemail if you set up voicemail with your carrier.4 However, you won't get a missed-call notification from your secondary number. Call Waiting works for incoming calls on the same phone number. To avoid missing an important call, you can turn on call forwarding and forward all calls from one number to the other. Check with your carrier for availability and to find out whether additional fees apply.

Cheers.