Gatekeeper Mode?

Hi all,
I’m facing a problem.
I have a SIP Account where my * box is registered !
When i try to make calls, the guy from the other company tells me that i’m trying to call with the gatekeeper mode!
he says : “You need to send call with direct Gateway mode.”

I don’t really understand why he wants that, because this box is working fine with another SIP account on another * box…

Someone can help me ???