The original note, as written at the time
What if we do international calls this way to save money and improve efficiency? Instead of running the entire call through the international line, the international line is only used to initiate the call. This is what I call the pinger. When the pinger is received, the receiving user is given the option to switch to a data connection in order to actually have the call.
Ongoing development
International calls done differently: the international line is used only to initiate the call, and then the two parties switch to a data connection to actually talk. The initiation is the pinger. When the pinger arrives, the receiving user is offered the switch — so even if their data is off, the call can still be established.
The economics are the reason and they are sound. An international trunk charges for duration; a data connection charges for volume, and a voice call is a very small volume. So use the expensive line for the one thing it is uniquely good at — reaching a number that is switched off — and the cheap line for the talking.
The elegance is the fallback. The pinger arrives as a normal signal, which means it rings a phone with no data, no app, and no credit. The recipient chooses whether to switch, which turns a technical trick into a decision the user makes.
The design problem is the seam. A call that begins as a trunk call and becomes a data call has to maintain the audio through the transition, and any gap is heard as a dropped call. The right approach is to establish the data leg first and hand over silently.
The second problem is trust. A call that asks you to switch connections is the exact shape of a fraud attempt, so the prompt must come from the dialler, not from a link.
Use the expensive line to knock. Use the cheap one to talk.