I'm just looking for a little clarification on something our client is looking for.
They currently have their AD infrastructure on prem and will shortly be migrating it into the cloud in its entirety. They want to adopt phone system using direct routing. is there anything stopping us setting this up using on prem accounts/ sub domains and then migrating those across when they complete their own migration or would this likely cause some crazy issue.
For direct routing setup you need azure AD, so you can sync object by using AD Connect tool and you can keep your OnPrem directory.
Direct routing can achieve on teams only mode and you need SBC to setup direct routing ( you can deploy in onprem DC or you can choose Virtual SBC as well, Azure have option to setup virtual SBC), if you are also opting phone license with MS then need to assign phone license, if you are opting external DID's or then you need to manually assign number to user.
I have recently setup this exact scenario for our users. Once the Direct Routing is setup (lots of PowerShell) it works fine. There are some quirks since the Admin GUI for Teams still needs some updating to see everything but using PowerShell solves this. Once you have your users replicated to Azure AD via Azure AD Connect you can start assigning numbers to users. This has no effect on their normal access..
If you would like some help/guidance, drop me a line.