It's a Hoax Re: phone snoops

[Michael Bailey]

the phone company can set various key sequences to certain values in
the voice switch.
Also, premise-based key systems and PBXs have the same capability.

While there's no hard and fast rule, sometimes 90# is used for one of
the many flavors of call forwarding.  The notion would be, if you
pressed that code while on the line with a fraudulent caller, the keys
they pressed after that would be recorded in the switch as the number
you wanted to forward to.

Thereafter, until deactivated, calls to your number would go to the
distant number.

 as scams go, it's not a very good one, imho.