Microsoft has announced that on April 30, 2019, Exchange Online UM will no longer offer direct SBC SIP access for third party PBXs.
This could force many customers to seek an alternative UM solution that integrates with Office 365 and their existing telephony infrastructure.
Microsoft lays out these migration options:
Active Communications can help.
We have a Unified Messaging system that provides a uniform user experience for PBX and Skype for Business.
This is the third party option suggested by Microsoft and offers a solution suited to both your PBX and Microsoft environment.