Set SIP server as Trusted Server?

I am having trouble setting up an inbound sip trunk. My service provider help desk stated “the reason the call is not being picked up by Asterisk is because the Asterisk Server is challenging the INVITE that we (the voip service) are sending it. You need to set the server as a trusted server, since if you challenge the network, we send the call to voice mail on our server.”

Can someone please push me in the right direction as to how I set my VOIP service providers server as a “Trusted Server” on my Asterisk Server?

Thanks in advance.

try with
insecure= very

in your sip-provider definition.