How two lines work on one phone
Which line handles calls, texts and data when your phone has two.
Both lines are on at once
With two lines set up, your phone shows two sets of signal bars. Both lines can receive calls and texts. You pick which one is the default for calls and messages, and which one is used for mobile data.
How SecondBars fits in
Leave your carrier as the default line for calls and texts, so your number works exactly as before. SecondBars is there for data: your phone uses it when your carrier has no coverage, either automatically or when you switch to it yourself.
One line carries data at a time
Mobile data runs over one line at a time. Automatic data switching lets the phone change which line that is without you opening Settings. The guides for iPhone and Android show where to turn it on.
Keep your carrier and your number.