modem: Log once dbus returns call was answered

Change-Id: Ia277b2eecdbf9969d77a688638837f9d9e1a44b6
diff --git a/src/osmo_gsm_tester/modem.py b/src/osmo_gsm_tester/modem.py
index 6a2a49a..59e2136 100644
--- a/src/osmo_gsm_tester/modem.py
+++ b/src/osmo_gsm_tester/modem.py
@@ -717,6 +717,7 @@
         assert self.call_state(call_id) == 'incoming'
         call_dbus_obj = systembus_get(call_id)
         call_dbus_obj.Answer()
+        self.dbg('Answered call %s' % call_id)
 
     def call_hangup(self, call_id):
         self.dbg('Hang up call %s' % call_id)