Q. I would like to know the opinion of the rav, shlita, what one should do if he was mekabel Shabbos at Plag HaMincha on Friday afternoon, and then remembered that he did not put on tefillin (for Friday)?
From Taz 600:2 it would seem to me that he can still put it on. Does the rav shlita agree? With or without a bracho?

A. Poskim rule that if one has only accepted tosefes Shabbos after plag and has not answered borchu yet, he can still don tefilin with a brocho, as his Shabbos acceptance can be restricted to only the prohibition of doing melochos (see prior answer.) We rationalize that he would not have accepted full Kedushas Shabbos had he known that he would lose the mitzvah of tefilin (see Taz 600: 2). However, Poskim disagree once he has accepted the Kedushas Shabbos by answering borchu or by reciting the tefilos of Shabbos. Some maintain that even if it is still daytime he should not put them on even without a brocho, since he may transgress various prohibitions such as zilzul Shabbos and bal tosif. (Piskey Teshuvos 31: 1 n. 3.)
Yalkut Yosef (261: 4), and Madanei YomTov (32) rule that they should be put on without a brocho. Horav Shlomo Miller’s Shlit”a opinion is similar. However, he should not don tefilin after the shkiah.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a