I set up a Voice over IP telephony account on my Android phone running Cyanogenmod 13, and later in the day I rebooted.
I was greeted with a popup "com.android.phone has crashed". It had a "more details" option which showed the exception's back trace:
java.lang.IllegalArgumentException: Invalid long: "firstname.lastname@example.org" at android.os.Parcel.readException(Parcel.java:1624) at android.os.Parcel.readException(Parcel.java:1573) at com.android.internal.telecom.ITelecomService$Stub$Proxy.getUserSelectedOutgoingPhoneAccount(ITelecomService.java:657) at android.telecom.TelecomManager.getUserSelectedOutgoingPhoneAccount(TelecomManager.java:463) at com.android.services.telephony.TelecomAccountRegistry.setupAccounts(TelecomAccountRegistry.java:510) at com.android.services.telephony.TelecomAccountRegistry.-wrap0(TelecomAccountRegistry.java) at com.android.services.telephony.TelecomAccountRegistry$1.onSubscriptionsChanged(TelecomAccountRegistry.java:286) at android.telephony.SubscriptionManager$OnSubscriptionsChangedListener$1.handleMessage(SubscriptionManager.java:397) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5456) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
When confirming the crash notification popup with "OK", the phone app got restarted and .. showed the popup again. I had about 0.5s to do something until the popup showed and blocked the complete user interface.
Adjusting the VoIP settings in Phone -> ⠇ -> Settings -> Calling accounts -> SIP accounts was not possible because this crashed, too.
I had to find the location of the file in which the settings were stored and get rid of them manually. With adb, find and grep I finally found that the VoIP accounts are stored in data/data/com.android.server.telecom/files/phone-account-registrar-state.xml.
While the crash popup was open, I removed the file with rm and then held down the power button until the phone rebooted. The error was gone - as were the VoIP account settings.
Clicking "OK" after removing the file restores the phone-account-registrar-state.xml state, so hard rebooting after file removal is a must.