You can't delete these, the apps are built into the firmware. Task Manager will harmlessly close them from running in the background, not delete them. I have no idea how you "reinstalled" one, perhaps you restored a shortcut to it from the apps drawer to the home screen.
Troubleshooting. First, click the "All apps" icon in the "favorites row." ensure you have "All Apps" button selected, not "Frequent" or "Downloaded." If you still don't see Phone or Contacts, something's wrong. Go to Settings | Apps, click "All" tab, and find Contacts, Dialer, Phone -- they should all be there. If you find them at the bottom marked "Disabled," someone's been messing with your phone.
If none of this works, you may need to Factory Reset with T-Mo's technical support reps' help, maybe just restart => hold the power button down until the popup appears, click Restart.
Have you read the Quick Start guide and User guide, available from the T-Mo support site?
Check my topic there. I'd download both manuals to your phone so you can refer to them any time.
Have you rooted and/or flashed this phone?