I don't think you're going to find any standards, what you are going to have to do is ensure that the positive outcome and the negative outcome are discernable.
I'm making an assumption that the reason you're using the haptics is because the device can be kept on the person, and the user would not need to hold the device in their hand to receive feedback on whether there request for help had gone through?
I would caution a couple of things: why would someone utilise this against calling a recognised emergency service? I have a background in law enforcement and, not always the case, but occasionally the fact that an operator can tell the caller to inform any perpetrator that the call is being recorded, the police are on route and they can hear everything being said, can be a deterrent. Furthermore, here in the UK (you'd have to check local policies if you're not UK based) people are advised, even if they can't talk, to dial 999 and when the operator answers - key 55, the operator will put the call through to the police as an emergency call, with similar actions taken as to the above.
Secondly; a cellular call is often more reliable than a data send, the reassurance provided by an app could be severely damaged if it doesn't achieve the aim of the application.
If there is another user case for actually doing this - I would ensure that the response is as identifiable as a result - i.e. a double buzz of the haptic is used far too often by many apps that provide notifications.
I would suggest you would look at a pattern that is:
- Easily identifiable
- Not easily confused with other notifications, including those within the same app
- Any ongoing notifications required to indicate that, perhaps, the app is trying again and then changes to a success or continues to indicate failure.
I hope this help.