Several reasons a phone why a phone constantly hangs up:
1. Memory storage could be running low
2. A lot of applications are running at the same time
3. Phone software is faulty

Common solution would include:
1. Uninstall apps and delete files you don't use to free memory space
2. Make sure to close an application once you're done with it
3. Upgrade software if an update is available
4. As a last resort, do a factory reset