I'm using a Lumia 640 Dual SIM running Windows 10 Mobile (10586.164), but am unable to choose which SIM card to call someone from within the People App now. You used to be able to press the down arrow and select which SIM card to call from, but now it doesn't matter where you press, it just dials the phone number without giving you any choice. As you can tell by the 'History' tab in the below screenshot, this is with the latest version of the People app.

Anyone else using a Lumia Dual SIM finding the same thing?

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