I see that whenever I use burner, I'm calling my burner number. I'm based in Canada, if I pick a US number as my burner number, will I be charged on my main phone plan for making US calls whenever I go through this app?
All calls inbound and outbound will display the burner number.
For instance, if your burner number is (333) 333-3333 and you get a call to that number from from (444) 555-1234, your phone bill will show a inbound call from (333) 333-3333 (not (444) 555-1234). Likewise, if you make an outbound call from Burner to (555) 555-5555 your bill would show an outbound call to your Burner number, not the (555) number.
As for the person you call / who calls you, their phone bill will show your burner number too.
In both cases, your burner line bridges you to the caller and vice versa, so the number showing up on both phone bills will be the burner number. What you get charged is up to your phone carrier and whatever plan you have. You can also use in-app calling if you're on Android, and that will make the calls go through your app instead of forwarding to your main cell.
Hope this makes sense! Let me know if you have any questions.