try

signal.als

@@ -292,13 +292,11 @@ check equivalent for 2 but 6 Message, 1..7 steps// Task 3.2: CHOOSE BOUND HERE
 
 pred two_rings {
  // Task 3.3: FILL IN HERE
-  some a, b: Address |
-    a != b and
-    some s, s': State |
-      s' in s.next and
-      s.ringing = a and
-      s'.ringing = b
+some disj caller1, caller2: Address |
+        some s: State |
+            s.ringing = caller1 and
+            after s.ringing = caller2
 }
-run two_rings for 3 but 8 Message, 1..8 steps // Task 3.3: CHOOSE BOUND HERE
+run two_rings for 3 but 6 Message, 1..10 steps // Task 3.3: CHOOSE BOUND HERE