best response confirmed by ThereseSolimeno (Microsoft)
Solution

@Bryan Hall, my take on it is that "Phone System" is the underlying PBX that provides the calling functionality to both SfB Online and MS Teams.

Teams Calling is the functionality within the Teams clients that allows you to leverage the underlying Phone System.

I don't believe the Advanced Communications SKU will give entitlement to Phone System, you need your users to have either an E5 or a Phone System Add-on license to use Phone System.

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