I see nothing special in the description they published on http://8did.com/en/features#voip
To forward calls through an SIP device, you must provide an SIP address instead of a forwarding phone number. An SIP address is like an email address, some of these are: 8885551212@216.52.11.11, 8885551212@mysip.mydomain.com, or even joe@mysip.mydomain.com.
This means that you should have a peer with host= matching their IP and a route for the number you’re forwarding to (8885551212 in the example above).