In short, yes.
How I know this is another question...
Rabbi Dr. Abraham Abraham writes in Nishmas Avraham [in the end of Volume 1, in the section listing the notes of Rabbi Eliezer Yehuda Waldenberg, the author of Tzitz Eliezer, number 2 *]
There, he disagrees with the Nishmas Avraham, who wrote that while a cheresh does not have to wear tefillin, we don't stop them if they do. Rav Waldenberg says that it is "קרוב לודאי" that the modern-day deaf [see later from the Kesav Sofer] have to wear tefillin, and therefore tefillin must be purchased for them.
- He (R' Waldenberg) quotes שו"ת בית שלמה (in או"ח צה) who ruled that the deaf who understand what is said to them, and can speak שפת עלגים (sign language? Google Translate doesn't know.....), their father must purchase tefillin for them.
- שבט סופר (in אה"ע כא) records a story that took place with his father, the Kesav Sofer (1815-1871 - Wikipedia), who was invited to tour a school for the deaf in Vienna. After the tour concluded, he said that the administration of that school must purchase tefillin for the deaf, because when they are schooled to be aware of their surroundings and are able to communicate, they are no longer considered to be a cheresh, and one must purchase tefillin for their use.
- He also quotes נחלת בנימין (see לא), who writes that a deaf person who will pray daily from a siddur can be used for a minyan for קדושה ברכו ותפילה (since those are rabbinic....he notes that there is room to argue on this point, but this is not its place אין כאן מקום להאריך. Maybe in a different question?).
To return to the original question:
if cochlear implants restore hearing and/or speech, then yes
, the person can be classified as "shome'ah" or "pike'ach".
With regard to hearing "ONLY when the implant is on," I would think that, based on the above, if he is able to communicate and be aware of the goings-on around him, he absolutely would be shome'ah/pike'ach, for sure when the implant is on, and probably also when it's off.
* In my (5 volume) edition of Nishmas Avraham, this appears on page שלט 379 in Volume 1 (אורח חיים). However, when I tried to link to it, I couldn't find it in the (4 volume) Hebrewbooks edition. If anyone finds a linkable copy, drop me a comment!