5.18.1.4. MFC-R2 Signals and MFCTA 
The MFC tables control the signal exchange and the meaning of the
forward and backward signals (physical-logical and reverse). Administer
these with the AMO MFCTA.
You can see in the table that signals have been kept free for national
use. Each country or provider uses these as needed. The terminal exchange
often requires the A number (ANI;identification) from the originating
exchange.
An SIU-X helps with the exchange of MFC signals.