How should a teaching-assistant deal with an angry student?
As a teaching assistant, you should not be abused. If a student becomes angry and abusive, you need to disengage and pass the problem upwards. While TAs participate in the grading of a course, the responsibility and authority for dealing with any significant grading dispute with a student lies with the professor, not with any TA. It's OK for TAs to deal with minor problems presented calmly (e.g., "You added up the scores and got the wrong total"), but major disputes and angry students are not things a TA can or should attempt to handle.
If you are confronted with an angry student, do not respond to the content of the dispute. If you do so, you may say something that can cause a major problem for yourself or the professor later. Instead, just keep repeating something along the lines of,
"Please calm down. This is something that you need to talk to the professor about."
If the student persists to the point where you feel threatened, then you have both the right and the obligation to get campus authorities involved. A person who is extensively verbally abusive may become physically violent as well, and you need to protect yourself.
Once you have disengaged from the student, contact the professor immediately and explain the situation. At that point, it is the professor's responsibility to deal with the student, though they may need more information and input from you in deciding how to proceed.
As mentioned in another answer, this falls into the general realm of dealing with angry people, not specifically students.
My suggestion in a situation like yours is to attempt to calmly disengage from the phone conversation as soon as possible. In many cases, you cannot make the student calm down.
I would then respond by e-mail to the student and CC the professor and any other relevant authority figure.
I've learned from personal experience that phone conversations or verbal interactions can be skewed later and mis-remembered by either or both parties. By responding by e-mail, you have increased the emotional distance from the student and provided a clear record in case of a later dispute. Moreover you have time to make sure your response is calm. I sometimes have trouble remaining calm when someone is yelling at me or verbally abusive.
Calmly explain your position and the grading policy. Attach a copy of the grading scheme from the beginning of the semester and make it clear in the e-mail that you do not appreciate the student verbally attacking you.
The student may dispute that they shouted or were verbally abusive, but because you CC the instructor, you have started a chain of evidence in case the student attempts to go over your head later.
When a student is calling your personal phone to shout and swear at you, you're past the point when you should deal with the situation on your own. Contact the relevant university authorities, and let the professor know what's going on. (Don't ask the professor to handle it; let the university handle it, but keep the professor informed about the situation.) Don't try to calm down or reason with the student on your own; it's not going to be successful, and he's already crossed the line into the area where the university does need to be involved.