Some users have reported an issue in which the iPhone repeatedly reboots in the middle of phone calls. Afflicted users reported that the phone simply restarts anywhere from 1 to 5 to 10 minutes into calls.
Potential fixes
Reset network settings. On your iPhone, open Settings, then navigate to “General” in the left-hand pane. Scroll down and tap Reset, then select “Reset Network Settings.” This will delete any stored WiFi passwords and other information, but may result in a more stable connection.
Restore, but not from backup. Although this fix is not universally successful, some users have reported relief via restoring the iPhone as a new device rather than from a backup. It appears that bad holdover data from iPhone backups can cause the reboot problem. Restoring as a new phone will delete contacts and other data, but may resolve this problem.
To do so, connect your iPhone or iPod touch to your computer, click “Restore” in iTunes, then choose “setup as new phone.”