There is a quirk with how Mircrosoft Teams routes voicemail messages. Any voicemail emails generated by Teams are not routed via MX records and instead are routed within Microsoft.
Therefore an issue occurs when Office 365 is locked down to only accept emails from Proofpoint in that the voicemail messages are rejected.
More information on this issue is detailed at https://docs.microsoft.com/en-us/skypeforbusiness/troubleshoot/hybrid-phone-system/voicemails-not-delivered.
As a workaround, a rule can be added to the Exchange Admin Center in Office 365 to allow voicemail messages from Microsoft. To create this rule:
1. Navigate to Exchange admin center > mail flow > rules
2. Click on "Create a new rule"
3. Create a new rule that:
Name: Teams Bypass
Apply this rule if: "The message type is"> include the message type > "Voice Mail"
Sender's IP address is in the range "126.96.36.199/14 or 188.8.131.52/14 or 184.108.40.206/18"
[please check Microsofts documentation for their latest Teams IP addresses - see https://docs.microsoft.com/en-us/office365/enterprise/urls-and-ip-address-ranges]
Do the following...
"Require TLS Encryption"
Chose a mode for this rule: Enforce
Check "Stop processing more rule"
4. Click Save
5. Ensure the completed rule is placed above the Proofpoint block rule
The completed rule will look like: