Are phone conversations between the two sexes permissible if for consultation, and within the limits of good manners?

Such act is impermissible since it renders hearts corrupt, and could lead to forbidden attachment. Allah, The Almighty, says: “and follow not the footsteps of the devil. “{Al-Baqarah/68}.

Is it permissible for the big brother to force his sisters to wear Hijjab (Islamic wear) if their father didn`t do his part in this regard ?

All Perfect Praise is due to Allah, The Lord of The Worlds

The father`s attention should be drawn gently and politely, and the sisters should also be reminded with good words since Allah, The Almighty, Says (What means): "O ye who believe! Save yourselves and your families from a Fire whose fuel is Men and Stones." [Al-Tahreem/6]. And Allah Knows Best.

What is the ruling on having slight discharges of menstrual blood during a day of Ramadhaan, and before Maghrib time ?

A woman who is certain that her menses have started during a day time of Ramadhaan is in a state of menstruation, and her fasting is considered broken at first sight of blood. However, she is rewarded for not breaking fast at the beginning of day time.

Is a woman`s neck an Awrah (private part)?

All Perfect Praise is due to The Lord of The Worlds.

It is an Awrah, and it is forbidden for her to reveal it before non-Mahrams (Marriageable men). And Allah Knows Best.