RealVNC problem.. I can connect in the same building but

but when I am elsewhere (obviously the reason why I bought this product), I get the time out (10600) error. I check the DNS when I hover the mouse over the VNC icon in the tray and the number is the same as always. As I said up front, when I am in the same building, I can VNC both directions, but when not in the same building, ie from a client’s office with high speed wireless, I cannot connect. Is their firewall blocking me?