It has always been a frustration to me in the Messages app that it doesn't really seem to know about my different SIMs. No indicator of which lines messages are coming from, no way to select which SIM to send messages from. Hopefully adding a Phone app has forced Apple to rethink this and not only provide this functionality in the new Phone app but bring it to Messages now as well. Could anyone running the macOS beta with a dual SIM iPhone speak to this?