Hybrid Solution Options
So what are your options when you have existing telephony systems or services in place, when they are unlikely to go away soon, and when you want to leverage existing investments while at the same time roll out Skype for Business Cloud PBX to at least some users? In a nutshell, you have two choices. Each involves putting all or parts of Skype for Business Server on prem and connecting Cloud PBX to it.
Option 1. For the situation I just described -- businesses with PBXs and presumably little or no Skype for Business Server on prem -- the path forward is Skype for Business Cloud Connector Edition (see related post, "Hybrid Cloud, Microsoft-Style, Garners Attention"). This is essentially a stripped-down set of Skype for Business Server workloads that a company adopting Skype for Business Cloud PBX deploys on premises or in a data center. Instead of going out over Microsoft's new PSTN Calling service, calls originating from Cloud PBX users flow to the Cloud Connector server, which in turn sends them to whatever PBX or gateways it's connected to on premises.
Option 2. The second option is for businesses that already have Skype for Business Server deployed on prem. The first assumption here is that the company has Skype for Business Server connected to a PBX or gateway, providing Enterprise Voice users homed to it with the ability to call out to the PSTN. The second assumption is that the company is now adopting or gradually migrating to Skype for Business Online. So some users are getting their business telephony from Skype for Business Server Enterprise Voice and others are getting it from Skype for Business Online Cloud PBX. In this latter scenario, Cloud PBX connects to the company's Skype for Business servers, which in turn link to the PSTN-connected PBX or gateway.