This is what I found from Yale's website:
http://www.law.yale.edu/documents/pdf/C ... offers.pdf
Accepting an Offer
When you are ready to accept an offer, you can either call the person who extended the offer to you, or in the case of larger law firms, you can call the Recruiting Department with your good news. If you have questions for the employer about your employment (i.e., salary, benefits, start and end dates, vacation issues), you should feel free to address them in your call. Although, after accepting a position, you will likely receive information from the employer setting forth the terms of your employment more specifically. If you accept a law firm job offer over the phone, it is usually not necessary to follow up with a letter. The employer typically follows up an acceptance with a letter confirming the acceptance and providing additional information about the position. With public interest employers, if you accept by phone, you should send a confirming email or letter.