Allowed-Denied numbers

I need to deny some extensions to call mobile number (all beginning with number 3 here in Italy)
I installed Custom Context module and create a Custom Context containing 3. dial rule
Next i set “Set All to:” to "Deny Rules"
As failover I set an Announcement saying “You are not allowed to this kind of calls” then hangup.
The extensions using this custom context effectively are denied to call mobile numbers.
Two questions:
Is this a regular way to proceed ?
What if I have to allow these extension to call a list of mobile numbers and deny all others ?
Perhaps should I set as failover a new Custom Context with a mobile numbers list as dial rule and Set all to: “Allow Rules” ??
Thank you very much