As a network administrator (Cisco switches, work with WAN providers, protocol analysis) I have an "area of responsibility" for the customer's VOIP project. The telecom group has primary responsibility for dial plans, voice mail and the monitoring tools that came with the Avaya system, but the traffic goes over the data network, so at some point I expect to be involved.
QoS has been implemented. I have not yet received any formal training, and most of the training offerings I have seen have had more to do with VOIP administration than data network related issues.
Network knowledge is great!... now try to research about the actual traffic being passed through your network. Become familiar with the logical and physical topologies of your VoIP infrastructure. Most training you find will definitely have to have QoS implementation and troubleshooting which should help you, not harm you.
Good whitepaper about VoIP: http://www.iec.org/online/tutorials/int_tele/index.asp
SIP Protocol information: http://www.voip-info.org/wiki/view/SIP
If you want to go deep into the actual traffic generated by VoIP, you might want to start looking at actual packet dumps. Here's a good resource (Search specifically for SIP and RTP):
http://wiki.wireshark.org/SampleCaptures
Also, ake close look at what people tend to ask about VoIP, specially in this great site: https://serverfault.com/search?q=VoIP
You already have the network knowledge, best thing to do is get familiar with the debugging and troubleshooting tools of the vendor's specific system.
Other than that, excellent Google skills... :)