Weird telephone number??? (646) 291-9400

I recently got a phone call on my cell phone from a friend. The caller
id said "caller unknown" but my bill reflected the number the call came
from was (646) 291-9400. When I looked up reverse records, it said the
number was an AT and T Wireless number, and when I tried to call it
back the day after the call and it said the number was disconnected.

My friend said she was in the UK and that she was calling from a cell
phone, however, this appears to be a New York telephone number. I also
doubted at the time that it was a "calling card" phone number, since
upon redial, there's usually an answer identifying it as the calling
card number.

Any ideas what this might be?