There is a cable: DE15-Y connector serial cable for RS232 GSM/GPRS&CDMA Serial MultiModem (Voice/Data). Can a voice call be made using this cable and the GSM/GPRS modem? What do I have to put at the other end of the Y cable?

Yes, a voice call can be made with this cable. It is designed to plug into a standard telephone handset.

In case you don't know, the standard product includes a cable without the "Y" branch for voice. The Y cable is an optional special order item.

The part number for the DE15-Y cable is 45880130

Here is a link to a diagram of this cable: