To recommend the appropriate additional licenses for a customer planning to replace their existing telephone provider with Microsoft Office 365 Enterprise E3, you need to address both the Cloud PBX capabilities and the ability for users to make and receive calls via the Public Switched Telephone Network (PSTN).
The two licenses needed are:
1. Cloud PBX User Subscription Licenses (USLs) – This enables private branch exchange (PBX) capabilities in the cloud, allowing users to control their call settings and connectivity.
2. PSTN Calling User Subscription Licenses (USLs) – This enables users to make and receive calls to and from external phone numbers (PSTN).