I'm trying to determine if it should be possible for a cell phone app to relay custom data along with a phone call. By "should be possible" I mean that I'm not trying to learn how to do it in iOS/Android/whatever, I'm just trying to determine if:
- It is permitted by standards: the standards for the metadata that initiates a phone call allow for it to be extended. (I found all kinds of information about high level phone call APIs and CDRs which are kept after a phone call is made, but nothing in the "this is the data header that leaves your phone when you make a call" category).
- I can be sure that my custom data will be forwarded intact by the cellular network nodes.
- This type of functionality is typically available to apps (from my research: some cellular functionality is implemented by firmware, other functionality has legal issues and its APIs are restricted to privileged users).
In short, what I would ideally like to hear is something along the lines of: "When you initiate a phone call, the phone generates this header. This part of the header can be used to send custom data, and cellular network nodes won't mess with it. The API for this should be available; for example on iOS/Android/whatever we have xyz()".