All good replies so far! Keep in mind there are hybrid options. We have a phone system that we offer to clients in our state (Indiana) right now (other states require expensive compliance changes that don’t make sense unless we sell a lot in one state), that offers integration with Microsoft Teams as an option. Teams then acts as an additional handset on an extension, which can also have desk phones, non-Teams mobile app, web phone, etc. and we can use all of our normal PBX features like call queues, auto-attendants, etc. while still letting users primarily use Teams if they want to. We just switched a church client over to this last week. About 35 users, a couple of phones without Teams, about 10 desk phones (those users mostly also have Teams), and the rest are all Teams-only. It’s a good mix of capabilities along with convenience of Teams if people are already using it.
You do have separate licensing costs with Microsoft for the phone system capability without a dial plan (which is added by the PBX) on top of the PBX cost. For charity pricing, Microsoft is currently at $3.20 per user for the Teams Phone System license to enable the capability. For businesses, it’s closer to $8. Then the phone system is integrated. But then you don’t pay Microsoft for their calling plan and deal with their number porting team (never had a good experience) or back-end features (limited, as discussed).
It’s also possible with some companies to add dialtone via Direct Routing (which is what our system uses above) without the separate full PBX features with the non-calling-plan licensing from Microsoft. Services vary on what features they offer. For us, we use it to link with our fully-featured PBX; for others, they can provide just the minutes or call capacity and numbers for an otherwise mostly-Teams system. Larger numbers of users do better with this usually, because of the complexity to set up and manage (the setup process with our integration still takes some time for every client because of all the Microsoft knobs to twist!)–a smaller team without complexity that can work directly with Teams probably wants to look at the full Microsoft system if they want to go that way.