You can do this easily by creating a caller ID routing rule (within the Advanced Call Routing area of your control panel menu). You can then choose what telephone numbers (or area codes) are diverted to your mobile. For example, you can reject all withheld numbers.
To enable the rule you will need to assign it to your telephone number. Please see this guide for more information.
Alternatively you could add a PIN access rule to the telephone number to ensure that only people that are aware of a PIN code. This is especially useful for routing paid support calls to out of hours engineers. Please see this guide for more help with setting up this feature.