If you intend to replace Skype for Business with Teams, in our business we now use SfB as our phone system, and you need to be able to show that Teams is viable as a Soft phone, with a suitable "Skin" on it to make it look more like a soft phone. Currently I cannot see people using teams as a soft phone, as we a) it doesn't look like a soft phone, b) we would need to set up teams to give the access to everyone. c) would we need a different team for each of our international offices?
You can setup Teams and limit the functions it uses by policies. If you want to use it just for Calling you can do that. The mobile app call tab acts like a very capable Soft Phone when enabled with the proper calling plan and phone system license. My users used it often and it works great. You don't have to have Teams setup to use chat and calling functions of Teams if you don't want to use those.