I am afraid the correct answer to your question is both yes and no.
Let me elaborate:
Is it possible to have multiple people broadcasting and receiving data at the same time? Yes
Can it be done with a voice server? Not really. It could be done, but it would require each member of your choir to have a steady connection, being physically located at equal distance from the server, being provided with an exact copy of the same machine and modem/router, being connected through the same landline and being serviced by the same ISP.
There's a concept, in communication, which is known as latency. Latency is the physical time a packet of data takes to move through the network from a point A to a point B. While the average ping for a voice server is between 30 and 80 msec, which means a packet takes between 30 and 80 msecs to travel from your client to the server and back (or, theoretically, from you to your friend, provided you have the same exact connection and are located at equal distance from the server, with both of your computers sending data through the same nodes), you also have to account for processing time, the time both the server and your own equipment will take to process and redirect each packet. Furthermore, the concept of resistance: the physical property of each material to resist the passage of current within it (directly influenced by the length of a cable), any network fluctuation or the physical distance between each client and the server would also have to be called into account while trying to shoot for perfect harmony.
The result would most likely be a mix-up of different voices, each singing a slightly desynchronized part of the song. It would, thus, both be unpractical if what you're aiming for is creating a good environment for rehearsing and would provide you with a totally unusable recording of your session.
If what you need is a way to record multiple voice layers for a track you're working on, might I suggest having your singers "phoning in" their part on different tracks and then merging them together via software?
I hope that answered your question, best of luck.