The VoIP switch is connected to a big 'server' box (labelled 'Voice Server') undeneath it (that's about as far as my expertise goes with VoIP.. sorry!)
Interestingly I did originally try plugging the Head's phone and PC into a splitter, then sending the cables from the splitter in the Comms Cab to the main switch and VoIP switch but they both refused to work that way. Tried swapping them around the jacks on the splitter, didn't do anything either. As soon as I put two identical devices (2 phones or 2PCs) to the same destination, it all works fine. Slightly confused, but never mind :)
we have a maxmisr phone system and using splitters did not work for us.
If the PC is not working through the phone then it will be down to the VLAN settings on your switch. The port will need to be in both VLANs so this usually means tagged in the Voice VLAN and untagged in the VLAN for the computer. That is how ours are all setup, you may also need some configuration on your DHCP server as well.