Don't throw away those old SIP phones.
In the old days of digital PBX phone systems, most vendors utilized proprietary technology for both hardware and software. This meant that if you purchased a Nortel PBX, you could forget about using your old Lucent phones. So, the decision to change PBX hardware, almost always involved a 100% forklift of the old system, including wiring. This is why, prior to VoIP, most businesses kept their office phone system for life. (Now you know why those PBX closets were always so dusty).