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.
Guys do you have the answer why does magicjack hangs up every after 33 seconds? because recently the problem always occuring. when i use magicjack, and when i call it says PENDIN ACK instead of active. then after 33 seconds it hangs up.. why is this?