To start with, all your contacts will be assigned to the mobile number you chose during setup. To change this, go to a contact and choose the default mobile number to be used for that person.
If you're just entering a phone number straight into the dialler, you'll be able to choose which mobile number to call from by selecting that mobile number at the top of the screen.
For incoming calls, there's an indicator to show which mobile number you're receiving the call on – this will be the first one or two letters of the name you assigned to each mobile number. So, if your labels are called Virgin Mobile and Business, the call indicator would be ‘V’ or ‘B’ (if they were Personal and Private they would be ‘Pe’ and ‘Pr’).