Yes, you can ... To disallow all calls, simply add * to the disallow register 05, as follows:
#0*
05
*
##
99
hangup
If you wish to alllow dialing of certain numbers, please use register 08, as follows:
#0*
05
*
##
08
(enter number 1 e.g. 305-558-5577)
##
08
(enter number 2 e.g. 786-221-5997)
##
(you can keep adding numbers to the allow register)
99
hangup
You should hear a beep after each programming line above.