If the local companies give you references from similar type businesses, with similar sizes, and parameters that is a good start, for further reference we will call these companies VENDORS.
VENDOR REFERENCES ARE WORTHLESS, BECAUSE NO VENDOR HAS EVER PROJECT MANAGED, INSTALLED, CUT OVER, PROGRAMMED, SERVICED, OR TROUBLESHOT A SINGLE PHONE SYSTEM IN THE HISTORY OF PHONE SYSTEMS. The vendor is of no importance the technician is the only real importance, followed by the project manager which is the only other factor.
Also get the direct email, cell, or other number of the tech in case you need to find someone who is of their caliber in the case that the vendor loses access to a tech of their caliber.