How The 90# Scam Occurs:
You are asked by someone to help troubleshoot a phone line or investigate calls placed from your line. They ask you to dial 9-0-# or to transfer to an outside line before hanging up. On some phone PBX (Private Branch Exchange) equipment dialing this gives the caller a free line to make a call where ever they would like for free.
More information on this and other common frauds can be found at: www.fcc.gov