exten => *67,1,Dial(ZAP/1/*67 ) exten => *67,2,Wait(3) exten => *67,3,SoftHangup(ZAP/1) exten => *67,5,DISA(no-password|from-internal)
*67 is the code used in the US to prevent the caller-id being displayed. In countries other than the US, this might differ. For example, in the UK we should dial the numbers 141. We should check that our providers offer this option and that we have the correct code.
If we read through the code, we can see that the script dials *67 through the first available PSTN line, performs a soft hang-up, which doesn't actually release the line, and then returns back a dial tone that will enable us to dial out to the number we want to call.