Interestingly, this is not some random app, but something that has been featured in Wired: https://www.wired.com/2015/09/turn-handful-phones-speaker-sy...

This is their webpage: https://ampme.com/. It says ©2019 Amp Me Inc. which could mean it was not updated since then. This is also when the app started charging money. Could it be that the authors sold the app and the developer account to scammers?

Also, not sure how to treat the accusation about easy to subscribe and hard to cancel. It seems the subscription can be canceled in iOS like all other subscriptions?

The app is also in Google store.