also worth double checking your netbook B's local firewall (if you have one enabled) - you may only have granted an exception to presenceVNC on the default port rather than 5901?

Also, how are you testing - are you VNC'ing into your LAN from a device on the net, or from another machine on your local LAN? I guess you are using your phone as a client? If so, install VNC client as well as server on the netbooks and try testing from A->B and vice versa to rule out any issue with the client setup on the phone and/or any WAN or ISP related issues (like the ISP blocking ports etc.).