Cant set up new iPhone XR

I order a new phone for my sister and she did a factor reset on the old phone so now we are trying to set up the new phone but we can't because we don't have access to enter in the 6 digit code that's its sending to the old phone. so now we are stuck with 2 phones that we can't use. We tired logging in on the computer to iMessage but its sill asking for the code sent to her old number. I tired calling sprint but they are currently close and she is leaving town in the morning, Can I get help on what to do?