Dial a Number Before Connecting the call

Hi Guys,
Im setting up a Overseas Calling Facility, in general to connect the call a number needs to be called first, it is a toll free number but unless it is dialled they cant call abroad.

How would i set asterisk up so it calls that number before it passes on the number the person has dialled?

Why does not simply prefixing the number not work?

Either the problem is trivial, or you have not fully specified it.

I think David is correct. You should do something like this
exten => s,1,Dial(DAHDI/1/18005551212w${EXTEN}

This will prepend your 800 number to the dialed number