What is the ruling on speaking on the phone with a woman out of necessity, and what is the ruling on using usurious money for education in cases of destitution?

Speaking with a non-mahram woman when necessary is permissible, subject to Shar'i controls, including not speaking in a soft or alluring manner. As for your use of the deposit saved for you and your siblings, there is no harm in it. You are entitled to the principal and the original deposit only. As for the usurious interest, you must dispose of it by spending it on the welfare of Muslims or giving it to the poor and needy, unless you are poor and in need and the original deposit is not sufficient for your needs (including your studies), in which case you may take from the interest an amount commensurate with your need.