Need to know more.
SIP or H.323 call?
Registered to a gatekeeper?
Is the far-end on the other side of a firewall?
What endpoint is the far-end using?
should be a firewall issue at your site. All needed ports opened and forwarded to your HDX?
Have a look in the FAQ here in the community, there is everything you need about all ports you need!
Or you have a look into the GUI of the HDX under Firewall, there you will see all needed ports, except 1720 TCP for signaling.
Definitely sounds like a firewall issue, or an issue with NAT'ing.