When you sign up your given a free number, that is the 05 number.......
You also get a number for texts and one other for conference calls.
You can simply ignore these numbers if you don't want, need or use them, it's what i do.
You can purchase/add as many phone numbers as you want against your Voipfone account but if you do they will all divert to the one phone registered against your account, if you purchase two geographical numbers for example and you want each number assigned to a different phone in two different locations then you will need to purchase two extensions. Simply register the first device against extension 1, the second against extension 2. Log in to your control panel and select where you want each number to ring.
You could get around paying for individual extensions by creating two individual Voipfone accounts and purchasing a geographical number against each account, this may cause you some hassle though if later down the line you decide on using additional features oh and you would have to top up each account separately when it comes to calling credit which could be a pain for some.