It depends on how your site is set up.
Go to another analogue extension and find out what number they give to people to dial them direct.
Usually it will be that you have a range of direct dial numbers purchased from your telco, i.e, 0207 383 4700 to 0207 383 4999.
Then the switch usually is set up to use the last 4 digits of the DD number. i.e., DD number 0207 383 4852 will feed to extension 4852 and that can be assigned as a vdn or a personal number.
Hope this helps