tweak CC cause for incoming call to unattached nr

So far we sent CC cause "Unassigned Number"

But the MSC doesn't trivially know whether the HLR has the number assigned or
not: any handset that is currently switched off would cause "Unassigned number"
to be displayed on the caller's handset.

Rather send a temporary failure cause code.

Send this cause code for all cases, because claiming that an assigned number is
unassigned is worse than rejecting an unassigned number with a temporary
failure.

Change-Id: Ia3d4f67b53fcc2654ff048fbc338e92cb763a095
diff --git a/src/libmsc/gsm_04_08_cc.c b/src/libmsc/gsm_04_08_cc.c
index ba6a197..1ec3342 100644
--- a/src/libmsc/gsm_04_08_cc.c
+++ b/src/libmsc/gsm_04_08_cc.c
@@ -1892,7 +1892,7 @@
 		}
 		if (!vsub)
 			return mncc_release_ind(net, NULL, data->callref, GSM48_CAUSE_LOC_PRN_S_LU,
-						GSM48_CC_CAUSE_UNASSIGNED_NR);
+						GSM48_CC_CAUSE_USER_NOTRESPOND);
 		/* update the subscriber we deal with */
 		log_set_context(LOG_CTX_VLR_SUBSCR, vsub);