The current bring your own device 30$ gives you 300 local minutes. Pretty sure you can have 2 BYOD on the same account as long as one subscriber on a your choice plan has a shared data bucket added to their plan. The cheapest is 20$ for 300 mb.
Some free apps use US numbers to send and receive sms. It can happen that an application you're using is actually causing those charges as the system is recognizing the sms to be international ones.
