If needing to stay on-prem I would always recommend Mitel MIVB as an enterprise solution.
For smaller companies, like under 75 extensions there are some good alternatives but once multi-site, multi-controller, multi-dialing paths enter the mix its Avaya or Mitel.
Programming can be a little complex but once it is running it is solid...
Mitel releasing new hardware for the MiVB service recently means it will maintain this as it's enterprise/hospitality service of choice.
Price wise comparable to Avaya, but waaaaaaaaaaaaay cheaper SWA than Cisco.
Saying that for an enterprise setup don't skimp on vMiCollab, MBG, MiCC, etc for the setup.
Get in close and good with your provider. Getting a couple good contacts there is good idea to bounce some questions off of as there is no way you will remember everything at the start!