I'm having problems with 3cxPhone. It works fine in CTI mode, but in 3cxPhone mode (in or out of office), I get the following error: 3CX Phone for Windows could not establish a connection to the 3CX phone System Web API. As a result features like Contacts and presence information will not be be available. you can still make and receive Phone Calls via SIP whilst we try and re-establish full functionality in the background. True enough to the error, I can make SIP calls in 3cxPhone mode, but this error pops up quite frequently and is annoying to say the least. Also true to the error, I get no presence information, contact info, or the ability to use the visual voice mail. I have installed the 3cxPhone app on the 3cx server and it does not seem to have any problems there. So it sounds like a firewall problem, but I have no idea how to fix it. I manually opened up the ports mentioned here: http://www.3cx.com/docs/firewall-router-configuration-voip/ and that did not seem to help. I'd love some guidance on what to try next.