Advice on cell phones for people new to the area.
NYC has four networks:
Verizon has by far the best coverage; you'll get reception whether you're underground, in an elevator, or on the 13th story. However, it comes with a premium price. And if you're on a family plan, it may be difficult to get a replacement if you lose or break your phone.
AT&T has an average signal. It's not as good as Verizon but better than T-Mobile. The customer service is crappy though. Columbia students have a 20% discount.
T-Mobile has several reasonably priced plans, but its signal on the Morningside Heights campus has a few dead spots. Notably, you won't get any reception while crossing in front of Butler Library. If you're looking for a pay-as-you-go plan, T-Mobile is probably the cheapest network.
Sprint has a spotty signal and poor customer service. In 2005 and 2006, Sprint made major improvements to its infrastructure (including adding new cell towers), changing the Columbia dead-zone to a reliable level of coverage. Some areas on campus still have a poor signal, especially within buildings. However, coverage outside of Columbia's campus is very strong. Columbia offers an 18% discount to students/faculty/alumni for phone purchases as well as a 5% discount on monthly plans.