BT no-longer publicise the 1xx three digit codes as a way of reaching BT services as they only work for direct subscribers connected to a BT line and using them for outgoing calls. This is no-longer universal so they now list only 0800 numbers as the official way to contact BT. Many of these are of the form 0800 800 1xx which are the same as the 1xx short codes.
The only exception to this is 100, which is still publicised by BT and connects the user to a BT operator if the customer uses BT for outgoing calls. The operator can then connect the user to a range of, usually expensive, chargeable services. Because this is a way to defeat call accounting and any barring or routing that may exist, and it is specific to BT, access to 100 is not configured on the PABX by default.
It is possible to configure the routing on the PABX web interface, but adding a number routing for “=100” and setting the destination as “ISDN”.