--- a/tmo/CSD-tests	Tue Nov 26 20:56:33 2024 +0000
+++ b/tmo/CSD-tests	Thu Nov 28 02:53:59 2024 +0000
@@ -226,9 +226,37 @@
 version, but some kind of law of chance: time of day, space weather, phase of
 the moon...
 
+Experiment 7
+============
+
+Earlier in Experiment 4 I observed that T-Mobile network does not pass through
+CSD call indication: when I dialed CSD calls from subscriber line A to
+subscriber line B, the destination MS would receive a voice call, while the
+calling side presumably invoked modem emulation IWF.  Just for the sake of
+completeness and due diligence, I decided to also test the case of ISDN-mode
+CSD as opposed to modem emulation, i.e., ITC set ot UDI instead of 3.1 kHz.  In
+this experiment I dusted off two of my legacy T-Mobile SIMs that aren't my
+"personal" one in the Pirelli DP-L10, put those two SIMs into a pair of FCDEV3B
+boards, and powered up both FCDEV3Bs simultaneously.  I then set AT+CBST=71,0,0
+on both MS and tested dialing calls from one to the other in both directions,
+both voice and CSD.  Results:
+
+* On voice calls, the destination MS once again received MT SETUP message
+  containing _no_ bearer capability IE.
+
+* ISDN-mode CSD calls were rejected by the network.  The MO call got as far as
+  CALL PROCEEDING message and channel assignment (the network assigned TCH/F9.6
+  to the MO leg), but then came a DISCONNECT message with cause 21 (call
+  rejected) and indicated location 0011 ("transit network").
+
+* I didn't test analog-mode CSD, as that mode was already tested earlier in
+  Experiment 4.
+
 NT mode: need for further debugging
 ===================================
 
+(This section was written following Experiment 6.)
+
 The flaky nature of NT mode calls for further debugging: since we see
 MS-initiated DISCONNECT on those tries when it fails, we really need to know
 exactly what is making the MS unhappy.  But this debugging is made difficult by
@@ -345,11 +373,13 @@
   unreliable.
 
 * When I tried replicating in the present time (2024-11) the just-described
-  result from many years ago, I had no success: see description of Experiment 4
-  above.  Now when I dial a CSD call from one of my legacy T-Mobile SIMs to
-  another, the receiving MS gets an MT SETUP message that contains no BC IE at
-  all, same as with calls coming from outside PSTN, the call rings as voice,
-  and gets assigned TCH/AHS like other speech calls.  And because the modem-
-  emulating CSD IWF in the MO direction initially emits silence while waiting
-  for the answering modem, silence is what the receiving phone hears in this
-  errant setup.
+  result from many years ago, I had no success: see description of Experiments
+  4 and 7 above.  Now when I dial a CSD call from one of my legacy T-Mobile
+  SIMs to another, the receiving MS gets an MT SETUP message that contains no
+  BC IE at all, same as with calls coming from outside PSTN, the call rings as
+  voice, and gets assigned TCH/AHS like other speech calls.  And because the
+  modem-emulating CSD IWF in the MO direction initially emits silence while
+  waiting for the answering modem, silence is what the receiving phone hears in
+  this errant setup.  (The just-described behavior happens when ITC is set to
+  3.1 kHz on the MO CSD call; if I change it to UDI which I never tried in the
+  past, the network rejects the MO call instead.)