There are both too many standards to choose from, and not enough standards from which to choose. Without same-vendor equipment, your interoperability will be limited, at best, to very basic features such as a complete transmission. More advanced features such as caller ID may not work when you use a multivendor solution.
Firewalls present a special challenge to feature sets and even basic transmission.
Most enterprise time division multiplexer networks still face the same multivendor interoperability issues even after 20 years of deployed networks. One IT manager completed a quick business case for adding voice over IP to his company network. He figured that considering each home office was already equipped with a broadband IP connection, these voice calls could ride for free. His discounted payback period was more than three years, so he has delayed his deployment until he can prove a better business case. The prognosis for voice over IP is not all bad, however.