Microsoft 365 Business Voice (without calling plan). They will now get Phone-system and AudioConferencing included.
https://docs.microsoft.com/en-us/microsoftteams/business-voice/whats-business-voice
Office 365 Business Essentials
Office 365 Business Premium
Office 365 E1
Office 365 E3
Office 365 F1
Microsoft 365 A1
Microsoft 365 A3
Microsoft 365 E3
Microsoft 365 Business