This is a common question I get since your PBX may not be in the same place as your mailbox servers.
Where do I put the Exchange UM components if I have a PBX in one location and an Exchange mailbox in another?
The typical rule of thumb to follow is:
The reason to keep Exchange UM server close to the Exchange mailbox server is the UM role talks to the mailbox server using RPC and over long distances the UM role does not behave well resulting in a poor end user experience.
If SIP Gateway/PBX is in one location and Exchange UM in another what bandwidth is needed?
The typical rule of thumb here is: