From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger addresses his son's difficulty regarding why the Gemara in Eruvin needs Michal bas Kushi to prove Shabbos is the time for tefillin, if women are already obligated via the lav of hishamer d'aseh. He resolves that the Eruvin sugya follows R' Elazar, who holds hishamer d'aseh is only an aseh, and further argues that the prohibition derived from hishamer may only apply to one already subject to the underlying positive commandment — since women are exempt from the aseh of tefillin, the lav would not apply to them either. He concludes the matter requires further resolution.
Source: Shu"t Rabbi Akiva Eiger §169