Many users took to the Apple discussion forums to report that the Phone app will crash or lock up when in the Phone app’s settings, in the Recent Calls list, and while making calls using Bluetooth headsets. Soft and Hard Resets have not fixed the problem, and those who have tried a factory reset have not experienced any changes either.
1, Update to iOS 10.3.2 if you haven’t already.
2, If the problem started after an update, the update itself may have installed incorrectly. If possible, restore your device to a backup made before the update was applied, then install the update again.
3, A proposed workaround suggests turning off Exchange contacts. You can do so by going to Settings > Contacts > Accounts, then tap your Exchange account(s) and toggle the Contacts slider off.