In hybrid scenario you might want to use PBX Online or/and use your existing on-premises PBX. Sometimes the regulation in your country is strict toward the VoIP traffic and that it can’t be used on-cloud and for this type of scenarios you’ll want to deploy the Hybrid topology and route all your VoIP traffic for online users toward your on-premises PBX ..

This can be easily managed with Skype for Business/Online and with few powershell commands you can control the call flow.