I dont really know the visonic tat that well, its one way wireless and I wouldn't touch it. But I dont know any panel with an app that doesn't work via the manufacturers cloud/servers. When a customer cancels their contract the panel id gets switched off at the cloud/server. So unless you build your own cloud and work out a way of making your panel talk to it you wont get it to work DIY. To talk to it through the USB you would need the manufacturers engineer software, not readily available.
I know that its not very green to tell you to bin it if it doesnt do what you want, I would suggest that you can either put an aftermarket dialler on it if you need notifications. Or bin it and get something that will do what you want. I would be cautious of any free app alarms though, servers cost a lot of money to run, and if someone is providing a service and not being paid for it, they are less in a rush to fix it when it goes wrong(They have already sold you the alarm kit there is no benefit to them rushing to fix something your not paying for)