Using a callwithus DID with FreePBX/Asterisk is very straight forward.
First you need to purchase the DID through your callwithus account. I am buying a number located in the UK.
Once you have purchased the DID you can click on the DID menu option again to check where the DID is being forwarded to. To make things easier on the FreePBX side we want to change this from the default.