A friend of mine received a telephone call recently from an individual identifying himself as an AT&T Service Technician (could also be Telus) who was conducting a test on the telephone lines. He stated that to complete a test they should touch nine(9), zero(0), the pound sign (#), and then hang up. Luckily, they were suspicious and refused.
Upon contacting the telephone company, they were informed that by pushing 90#, you give the requesting individual full access to your telephone line, which enables them to place long distance calls billed to your home phone number.
They dialed ‘0’ and asked the operator who confirmed that this was correct. They also checked Snopes.com and it applies to cellphones too.
Snopes says that it only applies to businesses, hospitals, government agencies, and other organizations that still use telephone private branch exchanges (PBXs) rather than Centrex lines to handle their calls.