Like BKNS27 said, have you replied "Yes" to the message confirming that you are porting/ transferring your old number to Public Mobile? Did you receive that message? Assuming you have responded properly to the message, and assuming that you have inserted your Public mobile simcard in the phone, All you have to do is restart the phone. On the other hand if you haven't received that prompt message to port, please restart the phone that has the old sim card inside. That happened to me I was wondering why I wasnt receiving any text message to confirm porting and all I had to do was restart. In short, restart all the phones that you are using while trying to activate your PM sim. (In my case I used two)Hope this helps, speaking from personal experience only. Anyway what steps have you taken so far?
