Many competitors have upgraded their designs (decades old, in some cases) to support SIP using bolt-on adaptors or “redesigning” their systems to be SIP lookalikes. Most compromise their designs to provide an upgrade path for existing customers. But unless you’re an existing customer who’s willing to pay the premium, it is an overly complex choice with a lot of added components and connectors. And those are potential points of failure in the long term.
You may be told that you can “upgrade” to SIP when you’re ready, just by purchasing the added adaptors. But since most companies only upgrade their telephone systems every 7-10 years, when the time comes, it only makes sense to upgrade everything. What if that time is now? Wouldn’t it make more sense to choose a system that isn’t limited to core technology that’s up to 20 years old, and can’t support SIP without added cost? Of course!