I've entered three cell numbers. (Its been a while)
Once one is entered, there is a "Verify Number" step.
This causes VOIPo to call the phone and ask for the access code.
(The code is on the web page when you click "Verify Number")
https://secure.voipo.com/beta/mobile

Then you can use any of these numbers to set to outgoing number people see for a second line on the second phone port.
https://secure.voipo.com/beta/upgrade_line

I also thought the primary number could be changed, but I cannot find a vPanel link . . .