US Mobile calls, like calls on all phone companies, are billed by the minute. For example, if you speak on the phone for 20 seconds or for 50 seconds, you will be billed for 1 minute. Calls that go to your voicemail are not considered billable, but the calls you make to your voicemail are billable.
For texts, you are billed for both incoming and outgoing text messages.
Note that at anytime if you feel that you are running low on texts, minutes, or data, you can top-up via the user dashboard.