So your business is running an old analog multi-line PABX that you've used and trusted for years, you've also just been letter dropped that the NBN is ready for service in your area, ok, so that means you have 18 months to migrate to an NBN service before Telstra cuts off your existing copper services, you know it's wise to move sooner rather than later because you don't want to get caught out and be without a phone service when everyone waits till the last minute, and there's not enough techs to go round.
You decide you better start talking to some RSP's (Retail Service Providers), or worse - you get one of those pesky door to door sales people, either way, during your conversations all of a sudden your ears prick up because you've just been told that your trusty existing PABX wont work, it's incompatible with the NBN and you need to replace it.