With a regular SIM card and an eSIM built in a phone, users can have two separate lines with two different plans inside a device. For example, a line can be used for work calls and the second line for calls from home. But if you talk to your wife and the boss calls the account number, the call from work will go to voicemail. Nor would it be a notification for the missed call. Call waiting will work if you receive a new call on a line already in use.