If you terminate the call, you always get an error on the next one. If the call terminates itself, you can always place another call. (This is the case with echo123.) When calling a phone with Skype Out, it is likely that the other party hangs down after the call. Sometimes Skype then detects the call being terminated. In this case, no problem, you can place another call. If Skype does not detect the call being terminated, the call goes on and on and you end up terminating it yourself (with the red button for example). This is the case were the sound problem arises.
So the workaround for people who have not (yet) used a more sophisticated one, or do not dare do so: avoid as much as possible terminating a call yourself. Always wait a while to see if it does not terminate by itself. If it does, no problem. If it doesn't, then, and only then, you have to completely close and restart Skype.
More information on https://wiki.ubuntu.com/SkypeHowto