I see that many are having trouble with keeping your old number. So I just whipped up this post which has step by step method including images for those visual audiences.
After you activate your account, proceed forward to the first step:
Step 1: Getting Your PAC (Port Authorisation Code)
Fill it out, and have fun with your GiffGaffThank YouI hope this helps. If any problems, or anything I need to edit, please reply