Although Location Services is an optional setting, iOS is enforcing the option when the device is locked.
We acknowledge this is a bug, and our developers are working with Apple to correct the issue.
In the meantime, there is a work-around:
In the iPhone (iOS) device settings...
- Select Privacy Settings
- then Real Softphone app
- Set Allow Location Access to Always
Your device will now allow calls to be answered, from a locked screen.