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Not all O365 licences need to be aligned to a Phone System Licence, but Yes, every user who requires voice enablement will requires a Microsoft Phone System licence. As an example, a customer tenant may include 1000 users, but you could choose only to enable a subset of those users for PSTN voice, say 800. All users aligned to a Phone System Licence would need to be matched to Microsoft Teams – Direct Routing enablement. In this scenario, all 800 would need to be declared. If these figures, do not align, then some identified users would not be recognised and would be unable to make/receive calls.