In short, magicJack must communicate with the company's servers, and an ACK is an acknowledgement that the communication was received. If an ACK is pending and not received, it is assumed that the communication failed, so it would try again for a set period/number of attempts before aborting. Unfortunately, that doesn't offer much to go on as it doesn't explain why networking issues are occurring. For that answer, I'd contact their technical support department so that they can troubleshoot the issue for you.

Best of luck.
John