Wave uses a non-proprietary version of SIP, so you may use your own SIP phones as long as they are true “SIP” and not a flavor of SIP that is proprietary/locked to another phone system.  Vertical does provide four different models of IP phones (the VIP-9800 series) which are feature-rich.  Wave also has “certified” 3rd party phones by Polycom, Aastra, and Spectralink.  Most any 3rd party SIP device should work with Wave.  Note that Vertical’s IP phones do not require a paid license to work with Wave, however, a 3rd party IP device does require an extra license to use with Wave.