90#on a telephone.
This is the Frequently Given Answer to that warning.
The warning that you have received, about pressing 90#, is a hoax message that circulates over electronic mail, faxes, and by word of mouth.
Hoax messages such as this circulate because people redistribute them to everyone that they know. This may be done with the best of intentions, but the information borne by these hoax messages is incorrect. It is in everybody's interest to stop the circulation of these hoax messages.
Please do not send on to anyone these hoax messages about dialling 90# on the telephone. If you have already sent the warning on to someone, send them a follow up message (pointing to this page if you like) explaining to them that it is actually a hoax and that they should not forward the hoax to people in their turn. There are some World Wide Web references at the end of this message that you can point them towards, as well, for verification that it really is a hoax.
This hoax warning began as a legitimate warning in a particular instutition in the United States in the early 1990s, but the versions that are in circulation these days, such as what you propagated, only partially resemble the original.
The original scam warned about in the original message only applies in the United States, and only to organisations with particular types of PABX that have been configured in a particular way. The essence of the original scam was the ability of these PABXes to connect two outside lines together, using call forwarding. This is why the Recall button is involved. The sequence R90# places the original call on hold, makes a new call to an outside line, dials the operator, and then connects the original call to the new call. The PABX thus needs to have been configured to allow an outside call to be forwarded to an outside line, which most PABXes do not allow, for obvious reasons.
However, the messages that circulate now have mutated. This is partly a Chinese Whispers effect. (The messages have been circulating by word of mouth since 1998, after all.)
In many cases, for example, the all important initial Recall keypress is absent. Without this all important first step, the sequence 90# is utterly meaningless whilst a call is in progress (unless you are talking to some other equipment that uses DTMF tones, such as a voicemail system or your bank's computer -- which wouldn't be the case here), even on the specific type of PABX that has been specifically mis-configured to allow the original scam to take place. You can press 90# however much you like during a call and it will be ignored. Warnings about doing so are bogus.
Other forms of the warning assure you that dialling *90# (notice the mutation that has substituted * for R) on GSM mobile telephones will erase your SIM chip, or that one should not pick up the telephone and dial 90# because it will allow unnamed "malicious hackers" to use your telephone line at some unspecified point in the future. None of these warnings have any sound basis in fact.
As mentioned above, the original scam only applied in the United States. What is perhaps most ironic when one sees this hoax message circulating in the U.K. (as - alas! - it does far too much) is that in the U.K., as everyone who has ever used a telephone should well know, 0 is not the code for the operator and # is not a valid telephone digit. Even if you had the right type of PABX in the U.K., and it had been misconfigured in exactly the right way, and even if you dialled the correct sequence from the original scam (i.e. with Recall), you would just connect the scam artist to the "number unobtainable" tone, which wouldn't be of much use to them.
People who circulate this hoax in the U.K. have obviously not thought about what they are reading before blithely forwarding it to other people. (Ironically, some people in the U.K. circulate variants of the hoax that talk about "AT&T Service Technicians". They obviously didn't stop and think about what they were reading at all.)
Please do not propagate this hoax. Instead help to stamp it out, especially outside of the U.S. where even a modicum of experience with using a telephone will tell one that the "warning" that it gives is nonsense. Tell people who send this hoax to you to stop sending it to people. If you have sent the warning on to others, out of misguided kindness, tell them immediately that it is a hoax and not to sent it on to any further people themselves.
The Korova Multimedia "Hoax Du Jour" essay from 1998 on The #90* Hoax.
The AT&T advice on how to avoid the original scam (not the hoax mutations).
The Internet Scambusters' article on The 90# Hoax (which they describe as an Urban Legend).
The CIAC HoaxBusters article on The 90# Hoax.
The AFU & Urban Legends Archives article on The 90# Hoax.
The Urban Legends Reference Pages article on the original scam and various related telephone scams.
A not completely serious article from the MiningCo on The 90# Hoax.
And finally, if someone insists that they heard this warning from a policeman/telephone company employee/system administrator/boss and that it therefore "must be true", tell them to read the Computer Virus Myths page on False Authority Syndrome.