if you follow this article to setup your integration with Exchange online (for voicemail)
you may kill your sfb federation. If this is a case, please note a following specificity of SFB:
when you set up Edge, you will be confronted with a choice, where to set your DNS. You can set it to external DNS, such as 126.96.36.199 or your internal DNS.
If you select external, you might need to define some hosts file entry like for FrontEnd pool, etc. Not ideal, right?
then you select internal DNS. However once you run
New-CsHostingProvider -Identity UMonline-Enabled $True -EnabledSharedAddressSpace $True -HostsOCSUsers $False -ProxyFQDN "exap.um.outlook.com" -IsLocal $False -VerificationLevel UseSourceVerification
you cut all federation. This is due to the fact that after this powershell , the edge will try to look for it's own SRV _sipfederationtls._tcp and because it is usually not defined internally, it will fail.
Solution is to check what is defined externally and in internal split DNS zone create exactly the same (SRV pointing to A record of to external public IP of edge access (SIP) interface.