kamailio.cfg: use set_contact_alias() instead of add_contact_alias()

(cherry picked from commit 66ed11c90e38d948fd75f908b3edd409bca74134)

etc/kamailio.cfg

@@ -758,7 +758,7 @@ route[NATDETECT] {
 			fix_nated_register();
 		} else {
 			if(is_first_hop())
-				add_contact_alias();
+				set_contact_alias();
 		}
 		setflag(FLT_NATS);
 	}
@@ -791,7 +791,7 @@ route[NATMANAGE] {
 	if (is_reply()) {
 		if(isbflagset(FLB_NATB)) {
 			if(is_first_hop())
-				add_contact_alias();
+				set_contact_alias();
 		}
 	}
 #!endif