Other phones like Polycom, Grandstream and Yealink all work but are not used for the main testing. If a client wants to use another phone for any reason don’t resist or talk them out of it. It is important to remind the client that any phone they want to use will work just fine, although we recommend Cisco SPA series phones.
Have recently been trying the cordless DP720 from Grandstream which has been better for a cordless solution then using ATAs (analog telephone adapter) and analog phones. Although ATAs retain most system features, they miss a couple like the hands free intercom ability.