It can reduce the overall bill, however; it depends on user preference.
2ndLine app is a separate infrastructure that requires only the Internet. So, if you are using mobile data, the use of the 2nd Line app will consume some data, though the app is optimized. This may be reflected under description like ‘mobile data usage’ on your primary phone bill. A call or message sent from (or received on) the 2ndLine app is not billed additionally on a phone bill.
We recommend using a stable WiFi more often to reduce the charges of mobile data usage.