Task: a smartphone/software/other device dropped an incoming call, called back this subscriber and connected to a conference call with a number ..?

Question

Telephony

How to make a smartphone / software / other device drop an incoming call, call back this subscriber and connect to a conference call with a number that the caller will dial from the keyboard of his phone or send in SMS. The required rate is available.

Answer 1

Those.
It is important to understand one detail here: at what stage do you want to implement this.
The first is the "correct" commercial one, when it is implemented on the operator's side, i.e. as B, an analogue of a switch is used (this, for example, may be a certain Call Back service, to which information on a call is routed in the operator itself - and such commercial platforms exist).
The second is when B is your SIM card and is in your own device and you implement the logic. Those. your device accepts, resets, and calls back to A. But in this case, DTMF to your B is not guaranteed so that B could call C.
Instead of DTMF, another business approach is usually used - USSD. A dials *111*C_NUMBER#, and the operator routes it to you on the logic. Your device sees A by default and C from the request. Then your hurdy-gurdy rings on A, A picks up, music is played to him, the hurdy-gurdy rings on C, C picks up the phone and your hurdy-gurdy connects them.
This is how commercial services are implemented.