Hosting Matrix is really easy. One part-time volunteer and a used server with backup is all that is needed for a few dozen users like I have.
Plus - think of all the money saved if you manage to ditch Teams altogether!
I don’t think you need anything bigger than that at headquarters. I recommend that field offices host their own if they have any kind of support for always on office equipment. If they don’t, they should still have their own homeserver hosted by another office or headquarters.
From matrix.org:
Matrix is really a decentralised conversation store rather than a messaging protocol. When you send a message in Matrix, it is replicated over all the servers whose users are participating in a given conversation - similarly to how commits are replicated between Git repositories. There is no single point of control or failure in a Matrix conversation which spans multiple servers: the act of communication with someone elsewhere in Matrix shares ownership of the conversation equally with them. Even if your server goes offline, the conversation can continue uninterrupted elsewhere until it returns.
This means that every server has total self-sovereignty over its users data - and anyone can choose or run their own server and participate in the wider Matrix network. This is how Matrix democratises control over communication.
By default, Matrix uses simple HTTPS+JSON APIs as its baseline transport, but also embraces more sophisticated transports such as WebSockets or ultra-low-bandwidth Matrix via CoAP+Noise.