When this issue came up with my sub, I felt that in the heat of the moment, especially if we have music playing, something like dropped keys or clicking fingers could be missed. Or some other circumstance prevents a tap out / dropping an object, like when she's in a straitjacket and being facefucked... so I purchased a wireless doorbell from Amazon. It works great.
While I think that the black ones look better, there isn't a lot of contrast between the frame and the button; I think that having high contrast makes it easier for the dog to see.
I have this one: https://www.amazon.com/SadoTech-Wireless-Doorbell-1000-feet-Adjustable/dp/B00FR4YQYK/ref=sr_1_5?dchild=1&keywords=wireless+doorbell&qid=1597447181&sr=8-5
Someone linked this: https://www.amazon.ca/SadoTech-Wireless-Doorbell-Operating-Batteries/dp/B00FR4YQYK/
But that's not going to work for OP, unless they're willing to run an extension cord to the garden / always remember to plug it in wherever they move (and hope that their patient doesn't ring while they're moving locations/don't have it plugged in).
The problem with any app is that it's always going to be prone to network connectivity on both ends or glitchiness on the devices or a dead battery or the person's ability to hit the app (it's amazing how often we lose our phones; they end up in the oddest places).
The wireless (door)bell system really is basically foolproof. You can get multiple receivers and if you're out in the garden you could just plug one in an outdoor socket while you are out there. I bought this one I think and it works great. They have other combos with more receivers and buttons if you wanted a receiver all over the house or buttons in multiple rooms. They can be set pretty loud as well.
I looked everywhere for an app like you are doing but there was nothing that was reliable enough in my view.
There's something about a simple button that has no purpose other than to call for assistance that works very well.
Beyond that, the only option I found was the emergency monitoring services which was overkill for my situation.
2 weeks ago we got the SadoTech model C (best seller in Amazon, excellent reviews and multiple models/configuration to choose from). I secured the transmitter to the wall and it's been working flawlessly since then. I only wish it had some memory to save the ringtone selection and volume if you disconnect the receiver, but other than that works great!
Yeah, there are cheap ones on Amazon that are wireless for under 20$
Here’s the doorbell I have. I use one of the holders for mini hand sanitizers to attach it to my lanyard.
If it’s too quiet you can buy an additional receiver too.
Let me know if you have any other questions!
I use a portable doorbell:
$20..... Maybe but one for each room.
Massive ring of buttons with that sign at the center.