Does a divorce take effect if the wife contacts her husband after he told her, "Do not call me again on the phone, or else I will divorce you," knowing that his intention was merely to threaten and not to divorce?

This wording is a threat of divorce, not a suspension of it. Therefore, divorce does not occur even if the wife does what she was forbidden from doing, just as the wife's phone call does not cause her to be divorced. One should be wary of using divorce pronouncements for threats.