This scam was being sent round the internet in 1998. Although the routine described may work on some PABX's it certainly won't work on residential lines. Below is one of the versions that was being posted on the usenet newsgroups. Many modified versions have been posted via e-mail.
This scam seems to be squarely targeted at businesses.
* High Priority **
On Saturday, 24 January 1998, Naval Air Station, Joint Reserve Base, New Orleans' Quarterdeck received a telephone call from an individual identifying himself as an AT&T Service Technician that was running a test on our telephone lines. He stated that to complete the test the QMOW should touch nine (9), zero (0), pound sign (#) and hang up. Luckily, the QMOW was suspicious and refused. Upon contacting the telephone company we were informed that by pushing 90# you end up giving the individual that called you access to your telephone line and allows them to place a long distance telephone call, with the charge appearing on your telephone call. We were further informed that this scam has been originating from many of the local jails/prisons. Please "pass the word".
More details on the above can be found at: http://korova.com/virus/hoax980212.htm
or The 9-0-# Phone Scam
Mobile Phone Variant
In January of 1999, I received a copy of the message that appears below. Note the similarities to the original:
If you get a phone call on your mobile from a Cellnet or Vodafone Engineer, telling you that they are doing a check on your Mobile Phone and that you have to press #90 or 09#. END THE CALL IMMEDIATELY This is a company who are executing a Fraud, they have developed and are using a device that once you have pressed #90/09#, will give them access to your SIM CARD details and allow to sell this information, so calls can be made at your expense.
This one is a HOAX.
I have spoken to Virus Bulletin, BT Cellnet (A supervisor in their customer services centre) and Vodafone (A senior engineer in their Security and Fraud section) in the UK, all have stated that this is a HOAX . The latter has written the following press release:
Vodafone's Statement Regarding Dialling 90#
There has been some concern about a potential fraud that can be incurred by users of mobile phones by dialling 90#.
Vodafone has been aware of these claims for some time and has made lengthy investigations but has been unable to recreate the effect or otherwise substantiate the claims.
In consultation with other networks, we believe this to be a false alert and an unsubstantiated claim.
Other experts have commented on this Hoax. See http://www.theaustralian.com.au/techno/4258422.htm