Skype for Business hybrid setup licensing question.

I have deployed an on-premise Skype for Business server in several steps. First, I got our in-house Skype working with IM, screen sharing, and phone calls both inside and outside. Then I set up federation so we can Skype with users on our clients' networks. The client is using Office 365 Skype so I moved our on-premise users to Office 365 so we can communicate with the 365 users. Here is where the problem comes in: when I move our users to 365 they lose the ability to make outgoing calls from Skype's dial pad. We can still IM, video chat, and call the other Skype users from their Skype contacts, but can't call our own cell phones for example. We have our own PBX that we want to continue to use for making calls, but we want to integrate it back into Skype so it works like it did on-premise. Do I need a license for Phone System to do this? Is there another way to get Skype to work with the other domain besides moving users to online? I spent a couple hours on the phone with Microsoft the other day and even they don't know what licenses i need.