In the case of Mitel we are talking two five-day courses – far from a walk in the park.
Which surely suggets that the phone system is overcomplicated? At the end of the day it has a simple job to fill, establish a voice circuit between two or more endpoints, with some frills like voicemail tacked on. If it takes 2 weeks training to configure that I'd say it was time to get a simpler system. Sounds like modern smartphones that do everything but make the tea, have no battery life to speak of, and yet most people don't use 90% of the "features" that their designers stuffed in. It's the downside of VoIP, glueing voice comms onto a protocol that was nevere designed for it, instead of using dedicated equipment that "just works".
The logical next step is to ask yourself whether you want to own the phone system at all. If the service provider is looking after it, why not think about renting it as an ongoing managed service instead of spending upfront on the hardware, software and licences?
You mean like we did 30 years ago when we just had the local Telco install phones in each office, each with a line back to the exchange where all the fancy stuff was done by experts who knew the kit? Sounds reasonable.