A mobile device initiates a call by sending a setup message to the network, which routes the message through various nodes to the destination device. The destination device sends a response message back through the network to the originating device to indicate if the call was accepted or rejected. If accepted, the network sets up the necessary connections to allow audio/video transmission between the devices for the duration of the call.

1. Call Setup
Basic Mobile Originating Call Diagram
Further reading: 3GPP TS 25.303, 25.331