DISA is great!
It allows you to call in to your Asterisk server, get a dial tone, and then dial back out as if you were using a normal extension on your system. I use this lots to make cheap international calls from my mobile phone.
You may also wish to route your DISA calls via A2Billing. If you’ve integrated FreePBX and A2Billing as described here it’s a simple case of changing one setting on your DISA setup in FreePBX.
All you need to do is change the ‘Context’ to ‘a2billing’ and you outgoing call will be passed to A2Billing to be routed. So you should end up with something like this is FreePBX –
One thing with DISA – make sure it’s secure! It’s a great feature but you don’t want other people making calls at your expense.