I've had the same problem as you with the volume. In my case, I found the answer.
MagicJack did a software update last month. This seems to have caused my volume control to malfunction as you described, setting limits at 1/4 on the low, and 3/4 on the high. To reset it to normal, go to the MJ Soft Keypad MENU button and choose "Volume/Headset Control", then move the speaker slider all the way to 0 and close the window. Now check your Windows volume control. Mine now works normally. Now reopen the MJ "Volume/Headset Control" panel and reposition it where it was. I found that it no longer set limits on my PC volume control and MJ works fine too.
Hope this helps,