Microsoft Teams

Question

We want to start using MTeams for our calls, internal and external, chat and video calls.

We have three businesses under a management company, can we have all the law firms under the one teams platform so that staff from across the board can communicate with one another or would you be required to have the same domain to sit in the same platform as everyone else (so email)

Also what is the cost to upgrading our 3 law firms to the business account which means we will require 3 reception capabilities along with staff, and porting our numbers across as well.

if someone could get back to me, that would be great.