I would say FIRST get a grip on color code and real stuff that you will need every time you work on a PBX. T1's, E&M circuits, Loop start and Ground start Trunks..etc. Learn to use a voltage meter. Couple that with real world experience with pulling structured cable, terminating it etc.. BTW this in itself will put you light years ahead of the IT guy.

You will need some networking experience in this day and age and gerneral networking concepts both voice AND data at this point.

Large PBX systems are very structured..or should be. A class in logic, in a local Comminity College, IMHO will help you in MANY ways.

After all this, work on an in house system and learn the hardware cold. Do some basic programming from the ground up by actually putting a system together. Going to a school for PBX training is really the way to go. My first I&M at Tadiran was 3 weeks of intensive training and quite frankly 3 guys failed the class. One guy got booted from simply not understanding how an E&M circuit was wired or the concept thereof.

Nothing beats real world experience though.