With the fixed dialling service (network service), you can restrict calls, and only allow calls to certain phone numbers. For example, you can only allow calls to family members.
Select Menu > Settings and Security.
Select Fixed dialling > On.
Enter your PIN2 code.
Add the desired numbers to the list of allowed numbers.
If you want to send a text message while the fixed dialling service is active, you need to add the message centre number to the list of allowed numbers.