Hi.
I would like to complete your project with your new PBX, just checked the manufacturer Mitel documentation and clearly it can works within VLANs, so then will be needed to have a low latency network (wired or wireless), if PBX can reach client devices within 200 ms, then system will work ok.
I have been network administrator for the last 5 years and I have implemented PBX systems.
Regards.