We usually use more than one SIM card on the phone. Because sometimes, we need to insert another SIM if one is out of balance. Then you have to open one and put another one. But nowadays, smartphones come with a dual sim facility, so you don’t have this hassle.
The Reno8 pro (china) phone is based on a dual SIM system so that you can use two nano-sized SIM cards simultaneously in this phone, and with this, the phone has been given a WiFi calling facility through which you can make phone calls without a SIM card. You can tell us through the comment box if you want to know more about the phone.