Dual-active SIM phones use two modems and allow you to receive calls on both numbers at once. If it's you who wants to make a call or send a text, Android has a standard SIM Management menu that lets you specify which SIM should be used for voice calls, video calls, messages and mobile data. You can either specify a particular SIM for each of these tasks, or leave the setting as 'Always Ask'. The data connection is where there seems to be a lot of confusion when it comes to dual-SIM phones. Whereas both SIM slots on some dual-SIM phones are capable of supporting 4G or 5G connections, you can use data connectivity on only one SIM at a time.  Unlike with calls and texts, typically speaking the data connection can't be on standby for both SIMs; you must specify which SIM you want to use rather than select one when prompted - though some phones will let you set them to switch to the other SIM when the first can't make a connection. By default, when you are using the data connection on one SIM and a phone call comes in to the other, it will pause the data connection on the first.