This is from an e-mail forwarded from a victim advocate friend--

"I received a telephone call last evening 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 the test I should touch nine(9), zero(0), the
pound sign (#), and then hang up.
Luckily, I was suspicious and refused.
Upon contacting the telephone company, I was
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.
I was further informed that this scam has been
originating from many local jails/prisons
DO NOT press 90# for ANYONE.
The GTE Security Department requested that I share
this information with EVERYONE I KNOW."

This applies to cell phones too!

[samhass]

Hmmm. Not sure. See http://www.snopes.com/fraud/telephone/jailcall.asp