Why do the phrases of the Talbiyah vary, such as: "Labbayka Umratan (Here I am for Umrah)," or "Allahumma Labbayka Umratan (O Allah, here I am for Umrah)," or "Labbayka Allahumma Umratan (Here I am, O Allah, for Umrah)"? Is each of these phrases based on sound evidence, or are not the wordings of such remembrances fixed (Tawqifiyyah)?

It is not impossible for supplications to come in more than one form, and the matter of the talbiyah is broad. The well-known talbiyah has been authentically reported from the Prophet, peace and blessings be upon him, and he did not object to those who added to it, such as the addition of "labbayka ilāh al-ḥaqq" (Here I am, O God of Truth), and the addition of Ibn Umar: "Labbayka wa sa‘dayka, wal-khayru fī yadayka, war-raghbā’u ilayka wal-‘amal" (Here I am, and at Your service, and all goodness is in Your hands, and all yearning is directed to You, and all action). It is recommended to vocalize one's intention, such as saying "Labbayka ‘umratan" (Here I am for ‘umrah) or "Labbayka ḥajjan" (Here I am for ḥajj), or "Allāhumma ḥajjatan lā riyā’a fīhā wa lā sum‘ah" (O Allah, a pilgrimage with no showing off in it, nor seeking reputation).