Is my defense of a sister who set "Allahu Akbar" chants as her phone ringtone, and her refusal to change them, considered a sin, especially given that I was accused of being argumentative when I tried to explain my viewpoint to the brothers?

One must adhere to gentleness and kindness when offering advice, as violence can lead to alienation. It has been previously explained that using Quranic verses and dhikr (remembrances of God) as phone ringtones may lead to prohibitions. If, in addition, they disturb others, then they should be avoided. The colleagues' disapproval of the girl has some validity, but perhaps they missed improving their approach.

As for their disapproval of those who are reserved in their interactions with male colleagues, that is inappropriate. Describing such interaction as "strictness" and "seclusion" carries corrupt meanings, for the boundaries of interaction between men and women and their Sharia-compliant regulations are clear. Therefore, defending this girl and these sisters is correct and in accordance with Islamic law.