A user did come up with a creative workaround to the lack of an external Siren. This will work without the HMS Subscription since it doesn’t use the HMS arming state as a trigger but uses routines on the sensors as a trigger. Since the sensors are all usable as triggers with the Hub without the subscription, it should work for your situation to activate a siren and notifications without a subscription, just not the Hub siren. In theory, this would turn it into a self monitored system, armed with a routine or rule outside of the HMS Monitoring tab, with a siren and push notifications.