Would you want a Butcher to do a Bakers' job? When it comes to IT networks and PC's, the IT guys are the best. If you enjoy messing around with networks and PC's then great, but if you need it to work and stay working, call the IT guys. That said, why do the IT guys try to install phone systems?
Yes, the IP in VoIP is a network protocol and, yes, the IT guys need to know and have a say in what runs and how it runs over the network, but the phone system is still very much the domain of the telephone guy.
In the last few years I've taken out more VoIP system, that were installed by IT guys, than I've installed myself. There was nothing wrong with the systems, in fact, they had considerable more functionality than the digital systems we replaced them with. It comes down to customer expectations and experience.
With VoIP, the expectation seems to be cost savings, although it is not the only expectation. The customer will expect the system to do all the things their old system did and more. Telecom experience will be needed to ensure expectations are met.