A call is a call, whether it involves the PSTN or whether it is internal to an organization or external. I say this upfront because this piece is about how Microsoft defines Teams callers now and how there’s a lot of room for growth.
Recently, Microsoft said it’s got 80 million active Teams phone users. Microsoft is including both VoIP and PSTN calls in its definition of phone users, which is entirely reasonable. With 270 million monthly active users of Teams and given how easy it is for any of these users to make a peer-to-peer call and thus become a “phone user,” 80 million active phone users is realistic.