--- a/doc/User-oriented-commands	Mon Feb 15 00:36:56 2021 +0000
+++ b/doc/User-oriented-commands	Mon Feb 15 02:01:01 2021 +0000
@@ -170,6 +170,51 @@
 Manipulating SIM phonebooks
 ===========================
 
+GSM SIM specs allow for several different phonebooks to be present on the card:
+
+* ADN (Abbreviated Dialing Numbers) is the main SIM phonebook.  Each SIM card
+  issuer decides how much storage space they allocate to ADN (how many records);
+  the SIM spec maximum is 254 records, and many issuers' SIMs do provide this
+  many records or close to this limit.
+
+* FDN (Fixed Dialing Numbers) is the "parental control" phonebook.  The FDN
+  phonebook can only be written to after authenticating with PIN2, and when it
+  is enabled (enabling FDN is done by "invalidating" ADN, an operation which
+  also requires PIN2), spec-compliant phones allow only numbers in FDN to be
+  called.
+
+* SDN (Service Dialing Numbers) is a service-provider-controlled phonebook: it
+  can only be written if you have special admin privileges (ADM authentication
+  method is card-vendor-dependent), and it is read-only to ordinary users.
+
+* MBDN (Mailbox Dialing Numbers) is a late addition to GSM SIM specs - it is a
+  special phonebook that stores the number for Voice Mail and other related
+  esoteric services.
+
+* MSISDN is a phonebook-like file that stores the subscriber's own phone
+  number(s).  Most classic GSM phones have a menu command for showing your own
+  number, usually called "My number" or something like that; this menu command
+  displays the first record stored in the MSISDN phonebook.  Most network
+  operators update this MSISDN record over the air (using special SMS-encoded
+  commands) when you activate service or get a new phone number without changing
+  your SIM, but this MSISDN store in the SIM also has some interesting
+  properties:
+
+  + Per the spec the MSISDN phonebook is writable by ordinary users, not just
+    admins, and the Mother's experience with real T-Mobile SIMs is that they do
+    indeed allow the user to write anything into MSISDN.
+
+  + Most SIM card issuers allocate multiple records for MSISDN, not just one.
+    It is not clear if ordinary end user phones would do anything useful with
+    the extra records if one were to write something there.
+
+fc-simtool provides a unified set of commands and data formats for working with
+all SIM phonebooks: all pb-* commands take the name of the phonebook to be
+operated on as their first argument.
+
+Last Number Dialed (LND)
+========================
+
 Manipulating stored SMS
 =======================