Export current call restrictions on DBus

This gives other components an idea what call restrictions currently
apply e.g. to display information like:

    Emergency calls only

on the lock screen.

src/dbus/org.gnome.Calls.EmergencyCalls.xml

@@ -37,5 +37,26 @@
         signal is meant as a hint (it might be emitted although the list of numbers is unchanged).
       -->
     <signal name="EmergencyNumbersChanged"/>
+
+    <!--
+        CallRestrictions:
+
+        The current call restrictions on this device with the currently
+        connected hardware and configured accounts:
+
+        Valid states:
+        - 0: disabled: The device doesn't support any calls e.g. no (voice capable) modem.
+             Typical state on modems and tablets.
+        - 1: emergency-calls-not-possible: The device supports emergency calls
+             but emergency calls are currently not possible. E.g. due to an internal failure.
+        - 2: emergency-calls-possible: *Only* emergency calls are currently possible
+             (e.g. because there's no SIM or the SIM is locked). "Regular" calls are
+             not possible.
+        - 3: none: The device operates normally. All calls types should be possible including
+             emergency calls.
+
+        Unrecognized values should be considered equal to emergency-calls-not-possible.
+    -->
+    <property name="CallRestrictions" type="u" access="read"/>
   </interface>
 </node>