It could be a 'runaway app' eating up your system RAM. When you are on the phone, it may just so happen that the phone call eats the last bit of RAM causing the phone to restart.
BACKUP YOUR PHONE BEFORE TRYING THE FOLLOWING!!! ALL DATA WILL BE WIPED DURING THIS PROCESS INCLUDING CONTACTS!!!
There is the possibility that the below will not correct your problem.
Try a factory reset on it and see if that helps the issue. To do this go to 'Settings' then to 'Security' then 'Factory Data Reset'. On devices with Jelly Bean, this is found under 'Settings' then 'Backup & Reset'.
After doing a Factory Reset, hold of on installing too many non-essential apps and see how the phone behaves. If it's OK, start installing apps slowly until you find one that makes the phone exhibit the rebooting issue.
Hope this helps!