What is the ruling on plucking eyebrows, given that there are female callers (to Islam) who do so and claim it is not forbidden?

It is not permissible for a woman to expose any part of her body in front of non-mahram men. The prohibition increases with the use of makeup. Plucking the eyebrows (al-Nams) is one of the major sins, due to the established curse of the Prophet, peace be upon him, upon those who do it. It is obligatory upon a Muslim to follow the ruling of the Sharia and avoid what displeases Allah.