Hi Miss Penny,
When making calls ensure that you are opening the TextNow application and calling out through the dial pad there. You can also try the following steps if you're still unable to make or receive those calls:
1. Open the Settings App.
2. Tap on Apps (or Application Manager).
3. Swipe over to the "All" heading (you will need to swipe from right to left across the screen).
4. Tap on TextNow (the list is sorted alphabetically).
5. Tap on Clear Cache.
6. Tap on Clear Data, then tap OK.
7. Tap on Force Stop, then tap OK.
8. Log back into TextNow.
Once you've completed that test out those calls again to make sure it's working!