You actually can turn off cellular network calling altogether, if you are willing to do that. Dial (star)#67# (or call 611 if it doesn't show up there) to see

You actually can turn off cellular network calling altogether, if you are willing to do that. Dial (star)#67# (or call 611 if it doesn't show up there) to see what number your voicemail center is. Then dial (star)21(star)1(that number)#. That will automatically forward all calls, at the network level, to your voicemail. To cancel this, dial #21#.