Setup DID to forward calls to ITSP

Hi I am new to freepbx and i am facing this problem.
I own a DID number from an ITSP_1
I want to forward incoming calls from ITSP_1 to an answering machine to ITSP_2.
ITSP_2 provided me a route at this form [email protected]
and nothing else.
I dont have a clue how do i have to configure a custom trunk to make this thing work.
Any suggestion would be appreciated