What I've come up with that should still work is set an outbound rule with a password in it.
91234 - 9 to dial out, 1234 is the password, then they dial the phone number. Just strip off the 9 & the password for the call to go through and you're done. Easy to change if needed too.
You can do it by extensions or by # of called digits or whatever you want as outbound rules apply.