iPhone Dual SIM Outgoing Call Logic = Roaming Nightmare

I am trying to express my disappointment on the dual sim features and open for discussion here but apple staff removed my topic and it seems they like to sweep questions under the carpet!

no wonder I found very limited topics regarding these troublesome issue.

my original post

This has been a painful experience for me while using Dual SIM in Singapore. My default voice line is my Singapore SIM, and my Malaysia SIM is secondary. But whenever I receive a missed call on the Malaysia SIM — even if I manually dial the number in the keypad—the iPhone still routes the call through the Malaysia line. This auto-switching results in painful roaming charges.

It’s not just “Call Back” behavior from Recents. It’s iOS actively remembering the last used line for that number and overriding my default SIM setting—even when I deliberately key in the number. That behavior feels absurd. What’s the point of having a “default line” if it can be ignored so easily?

To make things worse, despite my carrier officially supporting Wi-Fi Calling, it rarely activates. I only occasionally see the Wi-Fi Calling icon, even though I’m connected to a stable 2Gbps broadband. So I’m left wondering whether this inconsistency is due to iOS or the carrier — but either way, don’t blame the user!

This “last used SIM” logic might sound clever, but in real-world scenarios (especially roaming), it causes unexpected charges, confusion, and anger. Default SIM should mean just that — default, unless I choose otherwise.

I really wish Apple would give us an option to force calls to always use the default SIM unless manually changed. Is anyone else facing this problem? Any reliable workarounds beyond the ones I mentioned?