Java is your only development choice for the 7210, and from a Java application you cannot interfere (or, in most cases, even access) the normal functions of the phone. There is certainly no means of listening to a voice connection.
I was asked to write a program, which had to do this.
So I can't do this even if I use any specific Nokia's API functions?
It is sad.
The task is to create conference server. It has to recognize the second conference number from the [first] dial-in connection.
I don't know, what to offer to my customer..