I believe this is how to do it. Pick whatever number they give you in the options. Pick any it doesn't matter. Then after everything is said and done, as long as you're still logged in, go to your profile on the left side, click on "Transfer Number" the circular arrows chasing each other. Then enter the number you actually want (the one you want to port) then just follow the instructions from there. Just remember, depending on where it's coming from, you WILL need to leave your SIM card in the phone to get the "Hey, someone has asked to port your number" text and you will need to reply with yes. Another option is if you have a second phone, then put that OTHER SIM card that has the number you want to port in another phone so you can get that provider text to say yes while your PM SIM card sits in the phone you want to use. Make sense??
