All what you need to do is “open the door”
There is a table on the Quintum where you allow or disable calls from other gateways/gatekeepers.
All what you need to do, is setup an entry where you allow the second carrier gateway to access your equipment and send calls.
Or maybe you never setup the table to start with. In that case ,anyone could send you calls by knowing your I.P. address and the port used (1720 most of the times).
If you need help, write to firstname.lastname@example.org.