There are three ways to do this. Try these one at a time. These are listed in order of severity. If #1 fixes the issue, don't do #2, etc.
1. Soft Reboot your iPhone. Hold down the power button for 10 seconds until asked to slide to power down your iPhone. Do this, reboot it and try Voxie again.
2. Hard Reboot your iPhone. Hold down the power button and home button for 15 seconds and ignore the slide to power down message. Keep holding until your iPhone shuts off by itself. Reboot it and try Voxie again.
3. Reset your iPhone. Go to the iPhone's main Settings, then "General", then "Reset", then "Reset All Settings". However, after you do this, you will need to redo all your settings such as the connection to your network, ring tone, etc.