That’s a msg from Ivann:
You are right : they use DTMF tones. And yes, as their expert said, that server isn’t an ordinary one (PSTN as usual) “it’s also a modem connected to a terminal”. (The weirdest solution I’ve ever seen…)
It’s true, we have something like MS-to-MS schema. So we have one question (as I understand): on which channel are modems sending these tones (FACCH or ordinary speech frames)?
In some days I’ll be there and I’ll see this “invention” with my own eyes.