Ingate's SIParator/Firewall is required to connect IP-PBXs to the Exchange UM component of Microsoft Office 365 UM for Voice Mail and IVR. Ingate's SIParator provides the SIP connectivity to your PBX, handles the NAT/Firewall traversal and add a secure tunnel via TLS and SRTP to the Office 365 UM Service. Microsoft has interop-tested the Ingate SIParator and an application note is available for the setup.
... and more are continuously added. Contact Ingate to see if your IP-PBX is interoperable.