Hey Jackie,
The best thing to do to avoid any service interruptions, in my opinion, is first sign up for Public Mobile using a random number provided by Public Mobile during the sign up process. Then, when your account is created, sign in to it on a computer and click on "Profile" on the bottom left side of the page, then scroll down and click "Transfer Phone Number" on the right side of the page. Complete this process to port in your old number to replace the random number given to you by Public Mobile. This process can also be done through the phone app, but the locations of the buttons I described will be slightly different.
Edit: I forgot to mention that doing this will automatically cancel your plan with your old provider.
