--- a/doc/User-oriented-commands	Sun Feb 21 00:31:31 2021 +0000
+++ b/doc/User-oriented-commands	Sun Feb 21 01:05:05 2021 +0000
@@ -124,6 +124,11 @@
 themselves to networks, and they even use the first 5 or 6 digits of the IMSI
 to decide which network they should try connecting to first.
 
+It should also be noted that if your SIM has FDN (Fixed Dialing Numbers) enabled
+and the card implements GSM SIM specs to the letter, including the idiotic
+parts, then you will need to issue a rehab-imsi command before you can read the
+IMSI record - see the FDN section further in this document.
+
 sst
 
 Every SIM card is required to have an essential data record (an EF in technical
@@ -289,6 +294,38 @@
 starting record must be identified by number (SIM record numbers are 1-based);
 the ending record argument may be either a number or the "end" keyword.
 
+Enabling and disabling FDN
+==========================
+
+The Fixed Dialing Numbers (FDN) mechanism is normally disabled.  The protocol
+prescribed by GSM SIM specs is that FDN is enabled when the regular ADN
+phonebook is invalidated, and is disabled (unrestricted dialing allowed)
+otherwise.  fc-simtool provides commands for invalidating and rehabilitating
+ADN, thereby enabling and disabling FDN:
+
+inval-adn
+
+This command invalidates ADN and thereby enables FDN.
+
+rehab-adn
+
+This command rehabilitates ADN and thereby disables FDN.
+
+The SIM will only allow inval-adn and rehab-adn operations after you have
+successfully authenticated with PIN2 - see verify-pin2 command description.
+
+GSM SIM specs also stipulate a certain hack to prevent FDN-ignorant phones from
+making "forbidden" unrestricted calls: the specs stipulate that when a SIM
+powers up in an FDN-enabled state (ADN is invalidated), the "smart" logic in
+the SIM invalidates two essential files EF_IMSI and EF_LOCI (needed for GSM
+operation), requiring the phone (ME) to rehabilitate these two files at the
+beginning of every SIM session when FDN is in use.  The thinking must have been
+that if a given ME knows how to do these extra rehab-imsi, rehab-loci steps,
+then it also knows about FDN and will honor it.  Our answer: OK, whatever - but
+we do provide rehab-imsi and rehab-loci commands in fc-simtool.  These
+operations require only CHV1 access, thus PIN1 or no PIN at all depending on
+whether or not PIN1 is enabled - no need for PIN2.
+
 Last Number Dialed (LND)
 ========================