Call from external client with asterisk

Hi, I try to explain my problem.
I have some sip client registered on my asterisk pbx; my asterisk pbx is registered to a main asterisk pbx of the company with a virtual client.
I can call with a client on my asterisk an internal client of the company by managing the call with the virtual client but an internal client can’t call me.
How can I realize this? My sip client must be called as an internal client.
Can someone help me?

Thanks, Marco