When before Davening is one to place on his Tefillin?

[Various customs exist regarding when before Davening the Tefillin are to be worn. Some[1] wear the Tefillin prior to Birchas Hashachar.] The Rosh would recite the morning blessings until the blessing of “Oter Yisrael Besifara” without Tefillin and would then wear the Tefillin, and only afterwards say the blessing of “Oter Yisrael Besifara.”[2] [Others[3] wear it after Birchas Hashachar, prior to Karbanos. Others[4] wear it prior to Hodu, after Eizehu Mikoman.[5] The Chabad custom is to wear the [Tallis and] Tefillin prior to Eizehu Mikoman.[6]]

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[1] Siddur Yaavetz and majority of Siddurim; Piskeiy Teshuvos 46/7 footnote 71

[2] Admur 25/9; Michaber 25/3; Tur 25

The reason: This was done in order to give praise and thanks to Hashem in this blessing also on behalf of the Tefillin which are called a Peir, as the verse [Yechezkal 24/17] states “Pericha Chavush Alecha”. [Admur ibid; Taz 25/3; Tur]

[3] Kaf Hachaim 25/26 in name of Arizal in Shaar Hakavanos p. 1 and Rashash in Nehar Shalom, based on Kabala, and so is the order in Siddur Admur

[4] Kisvei Rav C.E. Beichavsky p. 232 that so is the custom of many;

[5] The reason: Some suggest the reason is because one is required to stand while reciting Karbanos just like a Kohen [see Admur Kama 1/14] and since the Tefillin are a Chatzitza for a Kohen, therefore it is not worn during Karbanos. However this reason would only apply for those who stand during the recital of Karbanos, while those who sit during its recital [as rules Admur in Basra 1/9] then this reason would not apply. [Kisvei Rav C.E. Beichavsky ibid]

[6] Hayom Yom 19^th Menachem Av; See Siddur Rav Raskin p. 68

The reason: Some claim to have heard from someone who heard from the Rebbe in Yechidus that the reason for this custom is in order to diminish the amount of time one wears the Tefillin, due to Hesech Hadaas. [Hearos Ubiurim O.T. 845 p. 76] Others question this reasoning. [Hearos Ubiurim O.T. 846] Others suggest the reason is because one is required to stand while reciting Karbanos just like a Kohen [see Admur Kama 1/14] and since the Tefillin are a Chatzitza for a Kohen, therefore it is not worn during Karbanos [and perhaps Eizehu Mekoman is no longer considered like brining Karbanos]. However this reason would only apply for those who stand during the recital of Karbanos, while those who sit during its recital [as rules Admur in Basra 1/9] then this reason would not apply. [Kisvei Rav C.E. Beichavsky ibid]